Merge #1553

1553: Added FAQ about SMTP Banner r=mergify[bot] a=arother

## What type of PR?
documentation

## What does this PR do?
Adding FAQ about SMTP Banner

### Related issue(s)
- closes #1368 

## Prerequistes
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

Co-authored-by: Andreas Rother <andreas@rother.org>
Co-authored-by: Dario Ernst <github@kanojo.de>

docs/faq.rst

@@ -255,7 +255,7 @@ correct syntax. The following file names will be taken as override configuration
 - `Nginx`_ - All ``*.conf`` files in the ``nginx`` sub-directory;
 - `Rspamd`_ - All files in the ``rspamd`` sub-directory.
 
-*Issue reference:* `206`_.
+*Issue reference:* `206`_, `1368`_.
 
 I want to integrate Nextcloud 15 (and newer) with Mailu
 ```````````````````````````````````````````````````````
@@ -355,6 +355,15 @@ down and up again. A container restart is not sufficient.
 
 *Issue reference:* `615`_.
 
+SMTP Banner from overrides/postfix.cf is ignored
+````````````````````````````````````````````````
+
+Any mail related connection is proxied by nginx. Therefore the SMTP Banner is also set by nginx. Overwriting in overrides/postfix.cf does not apply.
+
+*Issue reference:* `1368`_.
+
+.. _`1368`: https://github.com/Mailu/Mailu/issues/1368
+
 403 - Access Denied Errors
 ---------------------------