@jmtd sieve (via dovecot pigeonhole) is where I went from procmail. Been super happy.

@suihkulokki @jmtd As the Debian dovecot maintainer, I only just migrated from procmail to sieve in the past few weeks. Procmail had processed every single piece of email I received for over 25 years and I wasn't sure I would ever migrate! But it's good to be using sieve now, at least for dogfooding purposes.
Procmail being basically unconfined made it super flexible, but more than a little dangerous.

@jmtd fwiw, maildrop works fine for me, since years.

@jmtd Start here https://doc.dovecot.org/2.4.1/core/plugins/sieve_extprograms.html
But note that Sieve is, by design, more restrictive than procmail in this area. Modern mail server admins apparently don't want random users running random code on the servers. 🤷

@jmtd Exim Filter serves me nicely, but of course is unavailable unless you use Exim.

@jmtd IIRC, it stays in the queue and is periodically retried until "delivery" into the pipe is successful or the message times out and is bounced as undeliverable. I also have "unseen save Maildir/backup" at the top of my .forward so that I do at least save a copy of any messages that might get lost/mangled by later commands.

@jmtd You could always rewrite the envelope sender so that any bounces go into a suitable dead letter box.

@jmtd https://manpages.debian.org/bookworm/maildrop/maildropfilter.7.en.html : looks like it does the right thing: give tempfail back to mta if piping to a command fails, so the mta will retry later.