Manual should advise on mail server debugging

[mk-pmb]

Hi!

I'll start with a concrete example scenario as motivation, but the feature request will be more general.

I was trying to support $recipient (an uberspace user) who in turn was trying to support $sender (someone on the interwebs) about why $sender's mail server was unable to deliver mail to $recipient's uberspace mailbox. Unfortunately, even with guided advice, $sender was unable to provide a better error report than a screenshot of some part of $sender's office software where it said that the mail could not be sent, something about "action required" and "policy violation". It wasn't even clear whether that was an error message from SMTP refusal or there was a problem within the office software before even trying to connect to uberspace's SMTP.

Thus:

• The manual should tell how to check log files for failed attempts to submit mail.
• The manual should tell whether there are policy restrictions on what kind of mail is accepted or refused besides the spamfolder (which was disabled).