diff --git a/app/models/channel/driver/imap.rb b/app/models/channel/driver/imap.rb
index f12f8502f..1d04c11da 100644
--- a/app/models/channel/driver/imap.rb
+++ b/app/models/channel/driver/imap.rb
@@ -8,7 +8,7 @@ class Channel::Driver::Imap < Channel::EmailParser
   FETCH_MSG_TIMEOUT = 4.minutes
   EXPUNGE_TIMEOUT = 16.minutes
   DEFAULT_TIMEOUT = 45.seconds
-  CHECK_ONLY_TIMEOUT = 6.seconds
+  CHECK_ONLY_TIMEOUT = 8.seconds
 
   def fetchable?(_channel)
     true
diff --git a/lib/email_helper.rb b/lib/email_helper.rb
index 0f5c9f05d..4f37338d0 100644
--- a/lib/email_helper.rb
+++ b/lib/email_helper.rb
@@ -125,7 +125,7 @@ returns
           adapter: 'smtp',
           options: {
             host:      'smtp.gmail.com',
-            port:      25,
+            port:      587,
             start_tls: true,
             user:      email,
             password:  password,
@@ -148,7 +148,7 @@ returns
           adapter: 'smtp',
           options: {
             host:      'smtp-mail.outlook.com',
-            port:      25,
+            port:      587,
             start_tls: true,
             user:      email,
             password:  password,
@@ -171,7 +171,7 @@ returns
           adapter: 'smtp',
           options: {
             host:      'smtp.gmail.com',
-            port:      25,
+            port:      587,
             start_tls: true,
             user:      email,
             password:  password,
@@ -416,26 +416,6 @@ returns
     outbounds = []
     mx_domains.each do |domain|
       outbound = [
-        {
-          adapter: 'smtp',
-          options: {
-            host:      domain,
-            port:      25,
-            start_tls: true,
-            user:      user,
-            password:  password,
-          },
-        },
-        {
-          adapter: 'smtp',
-          options: {
-            host:      domain,
-            port:      25,
-            start_tls: true,
-            user:      email,
-            password:  password,
-          },
-        },
         {
           adapter: 'smtp',
           options: {
@@ -474,6 +454,26 @@ returns
             password: password,
           },
         },
+        {
+          adapter: 'smtp',
+          options: {
+            host:      domain,
+            port:      25,
+            start_tls: true,
+            user:      user,
+            password:  password,
+          },
+        },
+        {
+          adapter: 'smtp',
+          options: {
+            host:      domain,
+            port:      25,
+            start_tls: true,
+            user:      email,
+            password:  password,
+          },
+        },
       ]
       outbounds.concat(outbound)
     end
@@ -514,26 +514,6 @@ returns
 
   def self.provider_outbound_guess(user, email, password, domain)
     [
-      {
-        adapter: 'smtp',
-        options: {
-          host:      "mail.#{domain}",
-          port:      25,
-          start_tls: true,
-          user:      user,
-          password:  password,
-        },
-      },
-      {
-        adapter: 'smtp',
-        options: {
-          host:      "mail.#{domain}",
-          port:      25,
-          start_tls: true,
-          user:      email,
-          password:  password,
-        },
-      },
       {
         adapter: 'smtp',
         options: {
@@ -558,7 +538,7 @@ returns
         adapter: 'smtp',
         options: {
           host:      "smtp.#{domain}",
-          port:      25,
+          port:      465,
           start_tls: true,
           user:      user,
           password:  password,
@@ -568,6 +548,66 @@ returns
         adapter: 'smtp',
         options: {
           host:      "smtp.#{domain}",
+          port:      465,
+          start_tls: true,
+          user:      email,
+          password:  password,
+        },
+      },
+      {
+        adapter: 'smtp',
+        options: {
+          host:      "mail.#{domain}",
+          port:      587,
+          start_tls: true,
+          user:      user,
+          password:  password,
+        },
+      },
+      {
+        adapter: 'smtp',
+        options: {
+          host:      "mail.#{domain}",
+          port:      587,
+          start_tls: true,
+          user:      email,
+          password:  password,
+        },
+      },
+      {
+        adapter: 'smtp',
+        options: {
+          host:      "smtp.#{domain}",
+          port:      587,
+          start_tls: true,
+          user:      user,
+          password:  password,
+        },
+      },
+      {
+        adapter: 'smtp',
+        options: {
+          host:      "smtp.#{domain}",
+          port:      587,
+          start_tls: true,
+          user:      email,
+          password:  password,
+        },
+      },
+      {
+        adapter: 'smtp',
+        options: {
+          host:      "mail.#{domain}",
+          port:      25,
+          start_tls: true,
+          user:      user,
+          password:  password,
+        },
+      },
+      {
+        adapter: 'smtp',
+        options: {
+          host:      "mail.#{domain}",
           port:      25,
           start_tls: true,
           user:      email,
@@ -578,7 +618,7 @@ returns
         adapter: 'smtp',
         options: {
           host:      "smtp.#{domain}",
-          port:      465,
+          port:      25,
           start_tls: true,
           user:      user,
           password:  password,
@@ -588,7 +628,7 @@ returns
         adapter: 'smtp',
         options: {
           host:      "smtp.#{domain}",
-          port:      465,
+          port:      25,
           start_tls: true,
           user:      email,
           password:  password,
diff --git a/lib/email_helper/probe.rb b/lib/email_helper/probe.rb
index daf471e3f..31ae371aa 100644
--- a/lib/email_helper/probe.rb
+++ b/lib/email_helper/probe.rb
@@ -310,7 +310,7 @@ returns on fail
              else
                {
                  from:    email,
-                 to:      'emailtrytest@znuny.com',
+                 to:      'verify-external-smtp-sending@discard.zammad.org',
                  subject: 'This is a Test Email',
                  body:    "This is a test email from Zammad to verify if Zammad can send emails to an external address.\n\nIf you see this email, you can ignore or delete it.",
                }
diff --git a/test/integration/email_helper_test.rb b/test/integration/email_helper_test.rb
index 45aac3e10..501152ed5 100644
--- a/test/integration/email_helper_test.rb
+++ b/test/integration/email_helper_test.rb
@@ -41,7 +41,7 @@ class EmailHelperTest < ActiveSupport::TestCase
 
     assert_equal('smtp', map[:google_imap][:outbound][:adapter])
     assert_equal('smtp.gmail.com', map[:google_imap][:outbound][:options][:host])
-    assert_equal(25, map[:google_imap][:outbound][:options][:port])
+    assert_equal(587, map[:google_imap][:outbound][:options][:port])
     assert_equal(true, map[:google_imap][:outbound][:options][:start_tls])
     assert_equal(email, map[:google_imap][:outbound][:options][:user])
     assert_equal(password, map[:google_imap][:outbound][:options][:password])
@@ -89,6 +89,17 @@ class EmailHelperTest < ActiveSupport::TestCase
     assert_equal(email, map[1][:options][:user])
     assert_equal(password, map[1][:options][:password])
 
+    assert_equal('imap', map[2][:adapter])
+    assert_equal('imap.zammad.com', map[2][:options][:host])
+    assert_equal(993, map[2][:options][:port])
+    assert_equal(user, map[2][:options][:user])
+    assert_equal(password, map[2][:options][:password])
+
+    assert_equal('imap', map[3][:adapter])
+    assert_equal('imap.zammad.com', map[3][:options][:host])
+    assert_equal(993, map[3][:options][:port])
+    assert_equal(email, map[3][:options][:user])
+    assert_equal(password, map[3][:options][:password])
   end
 
   test 'provider_outbound_mx' do
@@ -101,18 +112,31 @@ class EmailHelperTest < ActiveSupport::TestCase
 
     assert_equal('smtp', map[0][:adapter])
     assert_equal('mx2.zammad.com', map[0][:options][:host])
-    assert_equal(25, map[0][:options][:port])
+    assert_equal(465, map[0][:options][:port])
     assert_equal(true, map[0][:options][:start_tls])
     assert_equal(user, map[0][:options][:user])
     assert_equal(password, map[0][:options][:password])
 
     assert_equal('smtp', map[1][:adapter])
     assert_equal('mx2.zammad.com', map[1][:options][:host])
-    assert_equal(25, map[1][:options][:port])
+    assert_equal(465, map[1][:options][:port])
     assert_equal(true, map[1][:options][:start_tls])
     assert_equal(email, map[1][:options][:user])
     assert_equal(password, map[1][:options][:password])
 
+    assert_equal('smtp', map[2][:adapter])
+    assert_equal('mx2.zammad.com', map[2][:options][:host])
+    assert_equal(587, map[2][:options][:port])
+    assert_nil(map[2][:options][:start_tls])
+    assert_equal(user, map[2][:options][:user])
+    assert_equal(password, map[2][:options][:password])
+
+    assert_equal('smtp', map[3][:adapter])
+    assert_equal('mx2.zammad.com', map[3][:options][:host])
+    assert_equal(587, map[3][:options][:port])
+    assert_nil(map[3][:options][:start_tls])
+    assert_equal(email, map[3][:options][:user])
+    assert_equal(password, map[3][:options][:password])
   end
 
   test 'provider_outbound_guess' do
@@ -124,18 +148,31 @@ class EmailHelperTest < ActiveSupport::TestCase
 
     assert_equal('smtp', map[0][:adapter])
     assert_equal('mail.zammad.com', map[0][:options][:host])
-    assert_equal(25, map[0][:options][:port])
+    assert_equal(465, map[0][:options][:port])
     assert_equal(true, map[0][:options][:start_tls])
     assert_equal(user, map[0][:options][:user])
     assert_equal(password, map[0][:options][:password])
 
     assert_equal('smtp', map[1][:adapter])
     assert_equal('mail.zammad.com', map[1][:options][:host])
-    assert_equal(25, map[1][:options][:port])
+    assert_equal(465, map[1][:options][:port])
     assert_equal(true, map[1][:options][:start_tls])
     assert_equal(email, map[1][:options][:user])
     assert_equal(password, map[1][:options][:password])
 
+    assert_equal('smtp', map[2][:adapter])
+    assert_equal('smtp.zammad.com', map[2][:options][:host])
+    assert_equal(465, map[2][:options][:port])
+    assert_equal(true, map[2][:options][:start_tls])
+    assert_equal(user, map[2][:options][:user])
+    assert_equal(password, map[2][:options][:password])
+
+    assert_equal('smtp', map[3][:adapter])
+    assert_equal('smtp.zammad.com', map[3][:options][:host])
+    assert_equal(465, map[3][:options][:port])
+    assert_equal(true, map[3][:options][:start_tls])
+    assert_equal(email, map[3][:options][:user])
+    assert_equal(password, map[3][:options][:password])
   end
 
   test 'z probe_inbound' do
@@ -525,7 +562,7 @@ class EmailHelperTest < ActiveSupport::TestCase
         adapter: 'smtp',
         options: {
           host:      'smtp.gmail.com',
-          port:      25,
+          port:      587,
           start_tls: true,
           user:      mailbox_user,
           password:  mailbox_password,