Implemented issue #2092 - Send postmaster email to sender if email is too big

.gitlab-ci.yml

@@ -172,6 +172,7 @@ test:integration:email_helper_deliver:
     - bundle exec rails test test/integration/email_helper_test.rb
     - bundle exec rails test test/integration/email_deliver_test.rb
     - bundle exec rails test test/integration/email_keep_on_server_test.rb
+    - bundle exec rails test test/integration/email_postmaster_to_sender.rb
 
 test:integration:facebook:
   <<: *test_integration_definition

app/models/channel/driver/imap.rb

@@ -206,6 +206,7 @@ example
     count                 = 0
     count_fetched         = 0
     count_max             = 5000
+    too_large_messages    = []
     active_check_interval = 20
     notice                = ''
     message_ids.each do |message_id|
@@ -226,13 +227,6 @@ example
       # ignore verify messages
       next if !messages_is_too_old_verify?(message_meta, count, count_all)
 
-      # ignore to big messages
-      info = too_big?(message_meta, count, count_all)
-      if info
-        notice += "#{info}\n"
-        next
-      end
-
       # ignore deleted messages
       next if deleted?(message_meta, count, count_all)
 
@@ -251,7 +245,24 @@ example
       end
       next if !msg
 
-      process(channel, msg, false)
+      # do not process too big messages, instead download & send postmaster reply
+      too_large_info = too_large?(message_meta)
+      if too_large_info
+        if Setting.get('postmaster_send_reject_if_mail_too_large') == true
+          info = "  - download message #{count}/#{count_all} - ignore message because it's too large (is:#{too_large_info[0]} MB/max:#{too_large_info[1]} MB)"
+          Rails.logger.info info
+          notice += "#{info}\n"
+          process_oversized_mail(channel, msg)
+        else
+          info = "  - ignore message #{count}/#{count_all} - because message is too large (is:#{too_large_info[0]} MB/max:#{too_large_info[1]} MB)"
+          Rails.logger.info info
+          notice += "#{info}\n"
+          too_large_messages.push info
+          next
+        end
+      else
+        process(channel, msg, false)
+      end
 
       begin
         timeout(FETCH_MSG_TIMEOUT) do
@@ -282,6 +293,11 @@ example
     if count.zero?
       Rails.logger.info ' - no message'
     end
+
+    if too_large_messages.present?
+      raise too_large_messages.join("\n")
+    end
+
     Rails.logger.info 'done'
     {
       result:  'ok',
@@ -420,7 +436,7 @@ returns
 
 check if email is to big
 
-  Channel::Driver::IMAP.too_big?(message_meta, count, count_all)
+  Channel::Driver::IMAP.too_large?(message_meta, count, count_all)
 
 returns
 
@@ -428,14 +444,13 @@ returns
 
 =end
 
-  def too_big?(message_meta, count, count_all)
+  def too_large?(message_meta)
     max_message_size = Setting.get('postmaster_max_size').to_f
     real_message_size = message_meta.attr['RFC822.SIZE'].to_f / 1024 / 1024
     if real_message_size > max_message_size
-      info = "  - ignore message #{count}/#{count_all} - because message is too big (is:#{real_message_size} MB/max:#{max_message_size} MB)"
-      Rails.logger.info info
-      return info
+      return [real_message_size, max_message_size]
     end
+
     false
   end
 

app/models/channel/driver/pop3.rb

@@ -136,6 +136,7 @@ returns
     count_all             = mails.size
     count                 = 0
     count_fetched         = 0
+    too_large_messages    = []
     active_check_interval = 20
     notice                = ''
     mails.first(2000).each do |m|
@@ -165,18 +166,28 @@ returns
         end
       end
 
-      # ignore to big messages
+      # do not process too large messages, instead download and send postmaster reply
       max_message_size = Setting.get('postmaster_max_size').to_f
       real_message_size = mail.size.to_f / 1024 / 1024
       if real_message_size > max_message_size
-        info = "  - ignore message #{count}/#{count_all} - because message is too big (is:#{real_message_size} MB/max:#{max_message_size} MB)"
-        Rails.logger.info info
-        notice += "#{info}\n"
-        next
-      end
+        if Setting.get('postmaster_send_reject_if_mail_too_large') == true
+          info = "  - download message #{count}/#{count_all} - ignore message because it's too large (is:#{real_message_size} MB/max:#{max_message_size} MB)"
+          Rails.logger.info info
+          notice += "#{info}\n"
+          process_oversized_mail(channel, mail)
+        else
+          info = "  - ignore message #{count}/#{count_all} - because message is too large (is:#{real_message_size} MB/max:#{max_message_size} MB)"
+          Rails.logger.info info
+          notice += "#{info}\n"
+          too_large_messages.push info
+          next
+        end
 
       # delete email from server after article was created
-      process(channel, m.pop, false)
+      else
+        process(channel, m.pop, false)
+      end
+
       m.delete
       count_fetched += 1
     end
@@ -184,6 +195,11 @@ returns
     if count.zero?
       Rails.logger.info ' - no message'
     end
+
+    if too_large_messages.present?
+      raise too_large_messages.join("\n")
+    end
+
     Rails.logger.info 'done'
     {
       result:  'ok',

app/models/channel/email_parser.rb

@@ -116,18 +116,15 @@ returns
     end
   rescue => e
     # store unprocessable email for bug reporting
-    path = Rails.root.join('tmp', 'unprocessable_mail')
-    FileUtils.mkpath path
-    md5 = Digest::MD5.hexdigest(msg)
-    filename = "#{path}/#{md5}.eml"
+    filename = archive_mail('unprocessable_mail', msg)
+
     message = "ERROR: Can't process email, you will find it for bug reporting under #{filename}, please create an issue at https://github.com/zammad/zammad/issues"
+
     p message # rubocop:disable Rails/Output
     p 'ERROR: ' + e.inspect # rubocop:disable Rails/Output
     Rails.logger.error message
     Rails.logger.error e
-    File.open(filename, 'wb') do |file|
-      file.write msg
-    end
+
     return false if exception == false
 
     raise e.inspect + "\n" + e.backtrace.join("\n")
@@ -486,6 +483,19 @@ process unprocessable_mails (tmp/unprocessable_mail/*.eml) again
     files
   end
 
+=begin
+
+  process oversized emails by:
+  1. Archiving the oversized mail as tmp/oversized_mail/timestamp_md5.eml
+  2. Reply with a postmaster message to inform the sender
+
+=end
+
+  def process_oversized_mail(channel, msg)
+    archive_mail('oversized_mail', msg)
+    postmaster_response(channel, msg)
+  end
+
   private
 
   def message_header_hash(mail)
@@ -783,6 +793,70 @@ process unprocessable_mails (tmp/unprocessable_mail/*.eml) again
 
     [attach]
   end
+
+  # Archive the given message as tmp/folder/timestamp_md5.eml
+  def archive_mail(folder, msg)
+    path = Rails.root.join('tmp', folder)
+    FileUtils.mkpath path
+
+    # MD5 hash the msg and save it as "timestamp_md5.eml"
+    md5 = Digest::MD5.hexdigest(msg)
+    filename = "#{Time.zone.now.iso8601}_#{md5}.eml"
+    file_path = Rails.root.join('tmp', folder, filename)
+
+    File.open(file_path, 'wb') do |file|
+      file.write msg
+    end
+
+    file_path
+  end
+
+  # Auto reply as the postmaster to oversized emails with:
+  # [ALERT] Message too large
+  def postmaster_response(channel, msg)
+    begin
+      reply_mail = compose_postmaster_reply(msg)
+    rescue NotificationFactory::FileNotFoundError => e
+      Rails.logger.error 'No valid postmaster email_oversized template found. Skipping postmaster reply. ' + e.inspect
+      return
+    end
+
+    Rails.logger.error "Send mail too large postmaster message to: #{reply_mail[:to]}"
+    reply_mail[:from] = EmailAddress.find_by(channel: channel).email
+    channel.deliver(reply_mail)
+  rescue => e
+    Rails.logger.error "Error during sending of postmaster oversized email auto-reply: #{e.inspect}\n#{e.backtrace}"
+  end
+
+  # Compose a "Message too large" reply to the given message
+  def compose_postmaster_reply(raw_incoming_mail, locale = nil)
+    parsed_incoming_mail = Channel::EmailParser.new.parse(raw_incoming_mail)
+
+    # construct a dummy mail object
+    mail = OpenStruct.new
+    mail.from_display_name = parsed_incoming_mail[:from_display_name]
+    mail.subject = parsed_incoming_mail[:subject]
+    mail.msg_size = format('%.2f', raw_incoming_mail.size.to_f / 1024 / 1024)
+
+    reply = NotificationFactory::Mailer.template(
+      template:   'email_oversized',
+      locale:     locale,
+      format:     'txt',
+      objects:    {
+        mail: mail,
+      },
+      raw:        true, # will not add application template
+      standalone: true, # default: false - will send header & footer
+    )
+
+    reply.merge(
+      to:            parsed_incoming_mail[:from_email],
+      body:          reply[:body].gsub(/\n/, "\r\n"),
+      content_type:  'text/plain',
+      References:    parsed_incoming_mail[:message_id],
+      'In-Reply-To': parsed_incoming_mail[:message_id],
+    )
+  end
 end
 
 module Mail

app/views/mailer/email_oversized/de.txt.erb

@@ -0,0 +1,12 @@
+[Unzustellbar] Nachricht zu groß
+Hallo #{mail.from_display_name},
+
+Ihre E-Mail mit dem Betreff "#{mail.subject}" konnte nicht an einen oder mehrere Empfänger zugestellt werden.
+
+Die Nachricht hatte eine Größe von #{mail.msg_size} MB, wir akzeptieren jedoch nur E-Mails mit einer Größe von bis zu #{config.postmaster_max_size} MB.
+
+Bitte reduzieren Sie die Größe Ihrer Nachricht und versuchen Sie es erneut. Vielen Dank für Ihr Verständnis.
+
+Mit freundlichen Grüßen
+
+Postmaster von #{config.fqdn}

app/views/mailer/email_oversized/en.txt.erb

@@ -0,0 +1,12 @@
+[ALERT] Message too large
+Dear #{mail.from_display_name},
+
+Unfortunately your email titled "#{mail.subject}" could not be delivered to one or more recipients.
+
+Your message was #{mail.msg_size} MB but we only accept messages up to #{config.postmaster_max_size} MB.
+
+Please reduce the message size and try again. Thank you for your understanding.
+
+Regretfully,
+
+Postmaster of #{config.fqdn}

db/migrate/20190806000001_setting_postmaster_send_reject_email.rb

@@ -0,0 +1,34 @@
+class SettingPostmasterSendRejectEmail < ActiveRecord::Migration[5.2]
+  def up
+
+    # return if it's a new setup
+    return if !Setting.find_by(name: 'system_init_done')
+
+    Setting.create_if_not_exists(
+      title:       'Send postmaster mail if mail too large',
+      name:        'postmaster_send_reject_if_mail_too_large',
+      area:        'Email::Base',
+      description: 'Send postmaster reject mail to sender of mail if mail is too large.',
+      options:     {
+        form: [
+          {
+            display: '',
+            null:    true,
+            name:    'postmaster_send_reject_if_mail_too_large',
+            tag:     'boolean',
+            options: {
+              true  => 'yes',
+              false => 'no',
+            },
+          },
+        ],
+      },
+      state:       true,
+      preferences: {
+        online_service_disable: true,
+        permission:             ['admin.channel_email'],
+      },
+      frontend:    false
+    )
+  end
+end

db/seeds/settings.rb

@@ -2582,6 +2582,33 @@ Setting.create_if_not_exists(
   frontend:    false
 )
 
+Setting.create_if_not_exists(
+  title:       'Send postmaster mail if mail too large',
+  name:        'postmaster_send_reject_if_mail_too_large',
+  area:        'Email::Base',
+  description: 'Send postmaster reject mail to sender of mail if mail is too large.',
+  options:     {
+    form: [
+      {
+        display: '',
+        null:    true,
+        name:    'postmaster_send_reject_if_mail_too_large',
+        tag:     'boolean',
+        options: {
+          true  => 'yes',
+          false => 'no',
+        },
+      },
+    ],
+  },
+  state:       true,
+  preferences: {
+    online_service_disable: true,
+    permission:             ['admin.channel_email'],
+  },
+  frontend:    false
+)
+
 Setting.create_if_not_exists(
   title:       'Notification Sender',
   name:        'notification_sender',

lib/notification_factory.rb

@@ -29,17 +29,26 @@ returns
 
 =end
 
+  class FileNotFoundError < StandardError; end
+
   def self.template_read(data)
-    template = File.readlines(template_path(data))
+    template_path = template_path(data)
+
+    template = File.readlines(template_path)
 
     { subject: template.shift, body: template.join }
   end
 
   def self.template_path(data)
-    template_filenames(data)
+    candidates = template_filenames(data)
       .map { |filename| data.merge(filename: filename) }
       .map { |data_hash| TEMPLATE_PATH_STRING % data_hash }
-      .find(&File.method(:exist?))
+
+    found = candidates.find(&File.method(:exist?))
+
+    raise FileNotFoundError, "Missing template files #{candidates}!" if !found
+
+    found
   end
   private_class_method :template_path
 

lib/notification_factory/mailer.rb

@@ -281,7 +281,7 @@ returns
     template = NotificationFactory.template_read(
       locale:   data[:locale] || Setting.get('locale_default') || 'en-us',
       template: data[:template],
-      format:   'html',
+      format:   data[:format] || 'html',
       type:     'mailer',
     )
 
@@ -292,6 +292,10 @@ returns
       template: template[:subject],
       escape:   false
     ).render
+
+    # strip off the extra newline at the end for the subjects of plaintext templates
+    message_subject.chomp! if data[:format] == 'txt'
+
     message_body = NotificationFactory::Renderer.new(
       objects:  data[:objects],
       locale:   data[:locale],

spec/lib/notification_factory/mailer_spec.rb

@@ -0,0 +1,93 @@
+require 'rails_helper'
+require 'ostruct'
+
+RSpec.describe NotificationFactory::Mailer do
+  describe '#template' do
+    context 'for postmaster oversized mail' do
+      let(:raw_incoming_mail) { File.read(Rails.root.join('test', 'data', 'mail', 'mail010.box')) }
+
+      let(:parsed_incoming_mail)  { Channel::EmailParser.new.parse raw_incoming_mail }
+
+      let(:incoming_mail) do
+        mail = OpenStruct.new
+        mail.from_display_name = parsed_incoming_mail[:from_display_name]
+        mail.subject = parsed_incoming_mail[:subject]
+        mail.msg_size = format('%.2f', raw_incoming_mail.size.to_f / 1024 / 1024)
+        mail
+      end
+
+      let(:en_expected_subject) { '[ALERT] Message too large' }
+
+      let(:en_expected_body) do
+        <<~BODY
+          Dear Smith Sepp,
+
+          Unfortunately your email titled \"Gruß aus Oberalteich\" could not be delivered to one or more recipients.
+
+          Your message was 0.01 MB but we only accept messages up to 10 MB.
+
+          Please reduce the message size and try again. Thank you for your understanding.
+
+          Regretfully,
+
+          Postmaster of zammad.example.com
+        BODY
+      end
+
+      shared_examples 'plaintext mail templating' do
+        it 'templates correctly' do
+          result = described_class.template(
+            template:   'email_oversized',
+            locale:     locale,
+            format:     'txt',
+            objects:    {
+              mail: incoming_mail,
+            },
+            raw:        true, # will not add application template
+            standalone: true, # default: false - will send header & footer
+          )
+          expect(result[:subject]).to eq(expected_subject)
+          expect(result[:body]).to eq(expected_body)
+        end
+      end
+
+      context 'English locale (en)' do
+        include_examples 'plaintext mail templating' do
+          let(:locale) { 'en' }
+          let(:expected_subject) { en_expected_subject }
+          let(:expected_body) { en_expected_body }
+        end
+      end
+
+      context 'German locale (de)' do
+        include_examples 'plaintext mail templating' do
+          let(:locale) { 'de' }
+          let(:expected_subject) { '[Unzustellbar] Nachricht zu groß' }
+          let(:expected_body) do
+            <<~BODY
+              Hallo Smith Sepp,
+
+              Ihre E-Mail mit dem Betreff \"Gruß aus Oberalteich\" konnte nicht an einen oder mehrere Empfänger zugestellt werden.
+
+              Die Nachricht hatte eine Größe von 0.01 MB, wir akzeptieren jedoch nur E-Mails mit einer Größe von bis zu 10 MB.
+
+              Bitte reduzieren Sie die Größe Ihrer Nachricht und versuchen Sie es erneut. Vielen Dank für Ihr Verständnis.
+
+              Mit freundlichen Grüßen
+
+              Postmaster von zammad.example.com
+            BODY
+          end
+        end
+      end
+
+      context 'unsupported locale, which defaults back to English locale (en)' do
+        include_examples 'plaintext mail templating' do
+          let(:locale) { 'UNSUPPORTED_LOCALE' }
+          let(:expected_subject) { en_expected_subject }
+          let(:expected_body) { en_expected_body }
+        end
+      end
+    end
+  end
+end

spec/models/channel/email_parser_spec.rb

@@ -997,4 +997,124 @@ RSpec.describe Channel::EmailParser, type: :model do
       end
     end
   end
+
+  describe '#compose_postmaster_reply' do
+    let(:raw_incoming_mail) { File.read(Rails.root.join('test', 'data', 'mail', 'mail010.box')) }
+
+    shared_examples 'postmaster reply' do
+      it 'composes postmaster reply' do
+        reply = Channel::EmailParser.new.send(:compose_postmaster_reply, raw_incoming_mail, locale)
+        expect(reply[:to]).to eq('smith@example.com')
+        expect(reply[:content_type]).to eq('text/plain')
+        expect(reply[:subject]).to eq(expected_subject)
+        expect(reply[:body]).to eq(expected_body)
+      end
+    end
+
+    context 'for English locale (en)' do
+      include_examples 'postmaster reply' do
+        let(:locale) { 'en' }
+        let(:expected_subject) { '[ALERT] Message too large' }
+        let(:expected_body) do
+          body = <<~BODY
+            Dear Smith Sepp,
+
+            Unfortunately your email titled \"Gruß aus Oberalteich\" could not be delivered to one or more recipients.
+
+            Your message was 0.01 MB but we only accept messages up to 10 MB.
+
+            Please reduce the message size and try again. Thank you for your understanding.
+
+            Regretfully,
+
+            Postmaster of zammad.example.com
+          BODY
+          body.gsub(/\n/, "\r\n")
+        end
+      end
+    end
+
+    context 'for German locale (de)' do
+      include_examples 'postmaster reply' do
+        let(:locale) { 'de' }
+        let(:expected_subject) { '[Unzustellbar] Nachricht zu groß' }
+        let(:expected_body) do
+          body = <<~BODY
+            Hallo Smith Sepp,
+
+            Ihre E-Mail mit dem Betreff \"Gruß aus Oberalteich\" konnte nicht an einen oder mehrere Empfänger zugestellt werden.
+
+            Die Nachricht hatte eine Größe von 0.01 MB, wir akzeptieren jedoch nur E-Mails mit einer Größe von bis zu 10 MB.
+
+            Bitte reduzieren Sie die Größe Ihrer Nachricht und versuchen Sie es erneut. Vielen Dank für Ihr Verständnis.
+
+            Mit freundlichen Grüßen
+
+            Postmaster von zammad.example.com
+          BODY
+          body.gsub(/\n/, "\r\n")
+        end
+      end
+    end
+  end
+
+  describe '#compose_postmaster_reply' do
+    let(:raw_incoming_mail) { File.read(Rails.root.join('test', 'data', 'mail', 'mail010.box')) }
+
+    shared_examples 'postmaster reply' do
+      it 'composes postmaster reply' do
+        reply = Channel::EmailParser.new.send(:compose_postmaster_reply, raw_incoming_mail, locale)
+        expect(reply[:to]).to eq('smith@example.com')
+        expect(reply[:content_type]).to eq('text/plain')
+        expect(reply[:subject]).to eq(expected_subject)
+        expect(reply[:body]).to eq(expected_body)
+      end
+    end
+
+    context 'for English locale (en)' do
+      include_examples 'postmaster reply' do
+        let(:locale) { 'en' }
+        let(:expected_subject) { '[ALERT] Message too large' }
+        let(:expected_body) do
+          body = <<~BODY
+            Dear Smith Sepp,
+
+            Unfortunately your email titled \"Gruß aus Oberalteich\" could not be delivered to one or more recipients.
+
+            Your message was 0.01 MB but we only accept messages up to 10 MB.
+
+            Please reduce the message size and try again. Thank you for your understanding.
+
+            Regretfully,
+
+            Postmaster of zammad.example.com
+          BODY
+          body.gsub(/\n/, "\r\n")
+        end
+      end
+    end
+
+    context 'for German locale (de)' do
+      include_examples 'postmaster reply' do
+        let(:locale) { 'de' }
+        let(:expected_subject) { '[Unzustellbar] Nachricht zu groß' }
+        let(:expected_body) do
+          body = <<~BODY
+            Hallo Smith Sepp,
+
+            Ihre E-Mail mit dem Betreff \"Gruß aus Oberalteich\" konnte nicht an einen oder mehrere Empfänger zugestellt werden.
+
+            Die Nachricht hatte eine Größe von 0.01 MB, wir akzeptieren jedoch nur E-Mails mit einer Größe von bis zu 10 MB.
+
+            Bitte reduzieren Sie die Größe Ihrer Nachricht und versuchen Sie es erneut. Vielen Dank für Ihr Verständnis.
+
+            Mit freundlichen Grüßen
+
+            Postmaster von zammad.example.com
+          BODY
+          body.gsub(/\n/, "\r\n")
+        end
+      end
+    end
+  end
 end

test/integration/email_postmaster_to_sender.rb

@@ -0,0 +1,213 @@
+require 'test_helper'
+require 'net/imap'
+
+class EmailPostmasterToSender < ActiveSupport::TestCase
+
+  setup do
+    Setting.set('postmaster_max_size', 0.1)
+
+    @test_id = rand(999_999_999)
+
+    # setup the IMAP account info for Zammad
+    if ENV['MAIL_SERVER'].blank?
+      raise "Need MAIL_SERVER as ENV variable like export MAIL_SERVER='mx.example.com'"
+    end
+    if ENV['MAIL_SERVER_ACCOUNT'].blank?
+      raise "Need MAIL_SERVER_ACCOUNT as ENV variable like export MAIL_SERVER_ACCOUNT='user:somepass'"
+    end
+
+    @server_address = ENV['MAIL_SERVER']
+    @server_login = ENV['MAIL_SERVER_ACCOUNT'].split(':')[0]
+    @server_password = ENV['MAIL_SERVER_ACCOUNT'].split(':')[1]
+
+    @folder = "postmaster_to_sender_#{@test_id}"
+
+    if ENV['MAIL_SERVER_EMAIL'].blank?
+      raise "Need MAIL_SERVER_EMAIL as ENV variable like export MAIL_SERVER_EMAIL='master@example.com'"
+    end
+
+    @sender_email_address = ENV['MAIL_SERVER_EMAIL']
+
+    @email_address = EmailAddress.create!(
+      realname:      'me Helpdesk',
+      email:         "me#{@test_id}@example.com",
+      updated_by_id: 1,
+      created_by_id: 1,
+    )
+
+    group = Group.create_or_update(
+      name:             'PostmasterToSenderTest',
+      email_address_id: @email_address.id,
+      updated_by_id:    1,
+      created_by_id:    1,
+    )
+
+    @channel = Channel.create!(
+      area:          'Email::Account',
+      group_id:      group.id,
+      options:       {
+        inbound:  {
+          adapter: 'imap',
+          options: {
+            host:           @server_address,
+            user:           @server_login,
+            password:       @server_password,
+            ssl:            true,
+            folder:         @folder,
+            keep_on_server: false,
+          }
+        },
+        outbound: {
+          adapter: 'smtp',
+          options: {
+            host:      @server_address,
+            port:      25,
+            start_tls: true,
+            user:      @server_login,
+            password:  @server_password,
+            email:     @email_address.email
+          },
+        },
+      },
+      active:        true,
+      updated_by_id: 1,
+      created_by_id: 1,
+    )
+    @email_address.channel_id = @channel.id
+    @email_address.save!
+  end
+
+  test 'postmaster reply with email on oversized incoming emails' do
+    imap = Net::IMAP.new(@server_address, 993, true, nil, false)
+    imap.login(@server_login, @server_password)
+    imap.create(@folder)
+    imap.select(@folder)
+
+    # put a very large message in it
+    large_message = "Subject: Oversized Email Message
+From: Max Mustermann <#{@sender_email_address}>
+To: shugo@example.com
+Message-ID: <#{@test_id}@zammad.test.com>
+
+Oversized Email Message Body #{'#' * 120_000}
+".gsub(/\n/, "\r\n")
+
+    large_message_md5 = Digest::MD5.hexdigest(large_message)
+    large_message_size = format('%.2f', large_message.size.to_f / 1024 / 1024)
+
+    imap.append(@folder, large_message, [], Time.zone.now)
+
+    @channel.fetch(true)
+
+    # 1. verify that the oversized email has been saved locally to:
+    # /tmp/oversized_mail/yyyy-mm-ddThh:mm:ss-:md5.eml
+    path = Rails.root.join('tmp', 'oversized_mail')
+    target_files = Dir.entries(path).select do |filename|
+      filename =~ /^.+_#{large_message_md5}\.eml$/
+    end
+    assert(target_files.present?, 'Large message .eml log file must be present.')
+
+    # pick the latest file that matches the criteria
+    target_file = target_files.max
+
+    # verify that the file is byte for byte identical to the sent message
+    file_path = Rails.root.join('tmp', 'oversized_mail', target_file)
+    eml_data = File.read(file_path)
+    assert_equal(large_message, eml_data)
+
+    # 2. verify that a postmaster response email has been sent to the sender
+    imap.select('inbox')
+    message_ids = imap.sort(['DATE'], ['ALL'], 'US-ASCII')
+    assert(message_ids.count.positive?, 'Must have received a reply from the postmaster')
+    imap_message_id = message_ids.last
+    msg = imap.fetch(imap_message_id, 'RFC822')[0].attr['RFC822']
+    assert(msg.present?, 'Must have received a reply from the postmaster')
+    imap.store(imap_message_id, '+FLAGS', [:Deleted])
+    imap.expunge()
+
+    # parse the reply mail and verify the various headers
+    parser = Channel::EmailParser.new
+    mail = parser.parse(msg)
+    assert_equal(mail[:from_email], @email_address.email)
+    assert_equal(mail[:subject], '[ALERT] Message too large')
+    assert_equal("<#{@test_id}@zammad.test.com>",
+                 mail['references'],
+                 'Reply\'s Referecnes header must match the send message ID')
+    assert_equal("<#{@test_id}@zammad.test.com>",
+                 mail['in-reply-to'],
+                 'Reply\'s In-Reply-To header must match the send message ID')
+
+    # verify the reply mail body content
+    body = mail[:body]
+    assert(body.start_with?('Dear Max Mustermann'), 'Body must contain sender name')
+    assert(body.include?('Oversized Email Message'), 'Body must contain original subject')
+    assert(body.include?('0.1 MB'), 'Body must contain max allowed message size')
+    assert(body.include?("#{large_message_size} MB"), 'Body must contain the original message size')
+    assert(body.include?(Setting.get('fqdn')), 'Body must contain the Zammad instance name')
+
+    # 3. check if original mail got removed
+    imap.select(@folder)
+    message_ids = imap.sort(['DATE'], ['ALL'], 'US-ASCII')
+    assert_equal(message_ids.count, 0, 'Original customer mail must be deleted.')
+
+    # final clean up
+    imap.delete(@folder)
+    @channel.destroy!
+  end
+
+  test 'postmaster reply with no email on oversized incoming emails' do
+    Setting.set('postmaster_send_reject_if_mail_too_large', false)
+    imap = Net::IMAP.new(@server_address, 993, true, nil, false)
+    imap.login(@server_login, @server_password)
+
+    imap.select('inbox')
+    message_count = imap.sort(['DATE'], ['ALL'], 'US-ASCII').count
+
+    imap.create(@folder)
+    imap.select(@folder)
+
+    # put a very large message in it
+    large_message = "Subject: Oversized Email Message
+From: Max Mustermann <#{@sender_email_address}>
+To: shugo@example.com
+Message-ID: <#{@test_id}@zammad.test.com>
+
+Oversized Email Message Body #{'#' * 120_000}
+".gsub(/\n/, "\r\n")
+
+    imap.append(@folder, large_message, [], Time.zone.now)
+
+    @channel.fetch(true)
+
+    # 1. verify that the oversized email has been saved locally to:
+    # /tmp/oversized_mail/yyyy-mm-ddThh:mm:ss-:md5.eml
+    path = Rails.root.join('tmp', 'oversized_mail')
+    target_files = Dir.entries(path).select do |filename|
+      filename =~ /^.+?\.eml$/
+    end
+    assert_not(target_files.blank?, 'Large message .eml log file must be blank.')
+
+    # 2. verify that a postmaster response email has been sent to the sender
+    imap.select('inbox')
+    message_ids = imap.sort(['DATE'], ['ALL'], 'US-ASCII')
+    assert_equal(message_ids.count, message_count, 'Must not have received a reply from the postmaster')
+
+    # 3. check if original mail got removed
+    imap.select(@folder)
+    message_ids = imap.sort(['DATE'], ['ALL'], 'US-ASCII')
+    imap_message_id = message_ids.last
+    msg = imap.fetch(imap_message_id, 'RFC822')[0].attr['RFC822']
+    imap.store(imap_message_id, '+FLAGS', [:Deleted])
+    imap.expunge()
+    assert(msg.present?, 'Oversized Email Message')
+    assert_equal(message_ids.count, 1, 'Original customer mail must be deleted.')
+
+    # final clean up
+    imap.delete(@folder)
+    @channel.destroy!
+  end
+
+  teardown do
+    Setting.set('postmaster_max_size', 10)
+  end
+end