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>
master
bors[bot] 6 years ago
commit 42953eb70b

@ -464,3 +464,7 @@ We **strongly** advice against downgrading the TLS version and ciphers!
.. _`681`: https://github.com/Mailu/Mailu/pull/681
.. _`698`: https://github.com/Mailu/Mailu/issues/698
.. _`unbound`: https://nlnetlabs.nl/projects/unbound/about/
A user gets ``Sender address rejected: Access denied. Please check the`` ``message recipient […] and try again`` even though the sender is legitimate?
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First, check if you are really sure the user is a legitimate sender, i.e. the registered user is authenticated successfully and own either the account or alias he/she is trying to send from. If you are really sure this is correct, then the user might try to errornously send via port 25 insteadof the designated SMTP client-ports. Port 25 is meant for server-to-server delivery, while users should use port 587 or 465.

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