F*EX use case: localdomain sender notifications

If you have F*EX users with email addresses not of your own domain (localdomain), then it can happend that the recipient's mailserver will reject the notification emails, when it runs in paranoid mode.

You can prevent this by rewriting the sender's address for the notification emails.
To do so, you have to:

Check if your fexserver is at least version 20200304

Execute:

     cd /home/fex/bin
     wget https://fex.belwue.de/download/fexmailforward
     chmod 755 fexmailforward

Install procmail and add to /home/fex/.procmailrc :

     :0 c
     * ^Delivered-To: fex\+.+_.+@
     * ! ^X-Forwarded-By: fex
     | /home/fex/bin/fexmailforward

Add to /home/fex/lib/fex.ph :

     # sender-From rewrite in notification emails
     $fex_sender = 'yes';

Add to /etc/postfix/main.cf (consult your local documentation if you are using an other MTA than postfix):
```
     recipient_delimiter = +
     
```
and restart postfix.