diff --git a/app/models/ticket/subject.rb b/app/models/ticket/subject.rb
index 12a307be3..2ab98943c 100644
--- a/app/models/ticket/subject.rb
+++ b/app/models/ticket/subject.rb
@@ -50,6 +50,8 @@ returns
 =end
 
   def subject_clean(subject)
+    return '' if subject.blank?
+
     ticket_hook         = Setting.get('ticket_hook')
     ticket_hook_divider = Setting.get('ticket_hook_divider')
     ticket_subject_size = Setting.get('ticket_subject_size')
diff --git a/spec/models/channel/filter/follow_up_possible_check_spec.rb b/spec/models/channel/filter/follow_up_possible_check_spec.rb
new file mode 100644
index 000000000..35ef3b945
--- /dev/null
+++ b/spec/models/channel/filter/follow_up_possible_check_spec.rb
@@ -0,0 +1,37 @@
+require 'rails_helper'
+
+RSpec.describe Channel::Filter::FollowUpPossibleCheck, type: :channel_filter do
+
+  context 'new_ticket follow_up_possible for group' do
+
+    let(:group) { create(:group, follow_up_possible: 'new_ticket') }
+
+    context 'ticket closed' do
+      let(:ticket) { create(:ticket, group: group, state: Ticket::State.find_by(name: 'closed')) }
+
+      it 'prevents follow up' do
+        mail = {
+          'x-zammad-ticket-id': ticket.id
+        }
+
+        filter(mail)
+
+        expect(mail[:'x-zammad-ticket-id']).not_to eq(ticket.id)
+      end
+    end
+
+    context 'ticket open' do
+      let(:ticket) { create(:ticket, group: group, state: Ticket::State.find_by(name: 'new')) }
+
+      it 'allows follow up' do
+        mail = {
+          'x-zammad-ticket-id': ticket.id
+        }
+
+        filter(mail)
+
+        expect(mail[:'x-zammad-ticket-id']).to eq(ticket.id)
+      end
+    end
+  end
+end
diff --git a/spec/models/ticket/subject_spec.rb b/spec/models/ticket/subject_spec.rb
new file mode 100644
index 000000000..a7e2ecc7c
--- /dev/null
+++ b/spec/models/ticket/subject_spec.rb
@@ -0,0 +1,51 @@
+require 'rails_helper'
+
+RSpec.describe Ticket::Subject do
+  let(:ticket) { create(:ticket) }
+
+  describe '.subject_build' do
+    it 'build subject based on new title' do
+      expect(ticket.subject_build('other title')).to eq("other title [Ticket##{ticket.number}]")
+    end
+    it 'build subject based on new title with ticket_hook_position left' do
+      Setting.set('ticket_hook_position', 'left')
+      expect(ticket.subject_build('other title')).to eq("[Ticket##{ticket.number}] other title")
+    end
+
+    it 'build subject based on new title without ticket_hook_position' do
+      Setting.set('ticket_hook_position', '')
+      expect(ticket.subject_build('other title')).to eq('other title')
+    end
+    it 'build subject based with forward argument' do
+      expect(ticket.subject_build('other title', 'forward')).to eq("FWD: other title [Ticket##{ticket.number}]")
+    end
+    it 'build subject based with reply argument' do
+      expect(ticket.subject_build('other title', 'reply')).to eq("RE: other title [Ticket##{ticket.number}]")
+    end
+  end
+
+  describe '.subject_clean' do
+    it 'cleanup subject with undefined string' do
+      expect(ticket.subject_clean(nil)).to eq('')
+    end
+    it 'cleanup subject with empty string' do
+      expect(ticket.subject_clean('')).to eq('')
+    end
+    it 'cleanup subject with long string which need to be truncated by [...]' do
+      expect(ticket.subject_clean('123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890')).to eq('12345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890[...]')
+    end
+    it 'cleanup subject with regular ticket#' do
+      expect(ticket.subject_clean("something [Ticket##{ticket.number}]")).to eq('something')
+    end
+    it 'cleanup subject with regular ticket# multiple time' do
+      expect(ticket.subject_clean("[Ticket##{ticket.number}] [Ticket##{ticket.number}] something [Ticket##{ticket.number}]")).to eq('something')
+    end
+    it 'cleanup subject with foreign ticket#' do
+      expect(ticket.subject_clean('something [Ticket#123456]')).to eq('something [Ticket#123456]')
+    end
+    it 'cleanup subject with some reply signs' do
+      expect(ticket.subject_clean('RE: RE: Re[5]: something [Ticket#123456]')).to eq('something [Ticket#123456]')
+    end
+  end
+
+end