Quoting Jim Jackson (jj@???):

> Just an aside - what are the alternatives to procmail?
> so far I've only found _maildrop_ as an in-line delivery filter.

Most-often mentioned alternative LDAs (local delivery agents) are
Maildrop, sieve[1], sortmail, and Dovecot LDA. (Me, I just persist in
using Procmail.)

The LWN.net article and comment thread (https://lwn.net/Articles/416901/)
covers some of this.

Note: If you look on Wikipedia, there's some confusion between LDAs and
MDAs (mail delivery agents), things that do mail transport like IMAP and
POP3 to -fetch- the mail. And, actually, LWN's Jon Corbet makes the
same regrettable category error. (Damned kids. Get off my lawn, and
don't kick my SMTP mail spool. ;-> )

[1] A filtering language rather than a utility as such. Specific
implementations are catalogued here: http://sieve.info/