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: