Add custom transport info

docs/u_e-postfix-custom_transport.md

@@ -0,0 +1,5 @@
+For transport maps other than those to be configured in mailcow UI, please use `data/conf/postfix/custom_transport.pcre` to prevent existing maps or settings from being overwritten by updates.
+
+In most cases using this file is **not** necessary. Please make sure mailcow UI is not able to route your desired traffic properly before using that file.
+
+The file needs valid PCRE content and can break Postfix, if configured incorrectly.

mkdocs.yml

@@ -65,6 +65,7 @@ nav:
       - 'Tagging': 'u_e-mailcow_ui-tagging.md'
       - 'Two-Factor Authentication': 'u_e-mailcow_ui-tfa.md'
   - 'Postfix':
+      - 'Custom transport maps': 'u_e-postfix-custom_transport.md' 
       - 'Whitelist IP in Postscreen': 'u_e-postfix-postscreen_whitelist.md'
       - 'Disable Sender Addresses Verification': 'u_e-postfix-disable_sender_verification.md'
       - 'Max. message size (attachment size)': 'u_e-postfix-attachment_size.md'