F*EX use case: localdomain sender notifications

If you have F*EX users with email addresses not of your own email domain (defined in fex.ph by @local_domains), then the recipient's mailserver will reject the notification emails, when it uses SPF.

You can prevent this by rewriting the sender's address for the notification emails:
user@other.do.main ==> fex+user_other.do.main@fex.mydomain

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

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

     # sender-From rewrite in notification emails for non-localdomains
     $fex_sender = 'yes'; 
     # or:
     # sender-From rewrite in notification emails for all domains
     $fex_sender = 'yes!';