922: Add FAQ about clients on port 25 generating non-helpful errors r=mergify[bot] a=Nebukadneza
## What type of PR?
documentation
## Prerequistes
- [x] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: place entry in the [changelog](CHANGELOG.md), under the latest un-released version.
Co-authored-by: Dario Ernst <dario@kanojo.de>
Co-authored-by: Ionut Filip <ionut.philip@gmail.com>