How to enable incoming plus addressing in Cpanel

[codeswitcher.livejournal.com]

Enabling plus addressing in Cpanel should be easy, what with all the layers of email filtering it presents the domain owner. The way to enable plus addressing is to filter incoming email on the basis of to whom its addressed. If you're fred@domain.tld, and want plus addressing to work, all forms of fred+arbitrarystring@domain.tld need to get routed to fred@domain.tld. That's a job for email filtering.

Alas, Cpanel presents the user with no way to filter incoming email on the basis of who it's really addressed for, i.e. the recipient specified in the envelope of the message. This is the address you really want for filtering, because without it, BCC and email list messages aren't delivered when you try to filter on the recipient. To add insult to injury, a "Envelope-to" field is added to messages at delivery -- after all filtering is done.

Cpanel is exim under the hood, and, frankly, the best thing about cpanel+exim is that it allows you to call out to some other real filtering program, if available.

And procmail, which is such a real filtering program, is on many linuxes by default. And it, in turn, can call perl so you can really get something accomplished.

So the answer to "How can I filter by the Envelope-To in cpanel+exim?" is "You can't. But you can call something that can."

( Instructions. Cut for epic length. )