Handling badly folded long Return-Path header gracefully

fetchmail: direct delivery mailbox/maildir using procmail/maildrop

You can make fetchamil use procmail (or maildrop) to deliver messages directly to mailbox (mailbox file or maildir) without sending it via SMTP. Your procmail script may even unfold Return-Path:.

see mdaoption in man fetchmail

I have used similar solution with custom per email account procmail scripts.

---

~/.fetchmailrc

poll pop.provider.net proto pop3
  user "jsmith" with pass "secret1"
  mda /usr/bin/procmail -m .procmailrc-provider.net

~.procmailrc-provider.net

 ########
 # Debugging settings
 ########
 # log all dignostic info to ~/procmail.log
 LOGFILE=procmail.log
 # log abstracts of delivered messages
 # Subject:`,`From:`, size, final destination 
 LOGABSTRACT=all
 # Turn on extended diagnostic
 VERBOSE=on
 ########

 # deliver everttyting to maildir (tailing /) located in directory ~/.maildir 

:0
.maildir/

Debugging:
fetchamil: -v (or even -v -v) command line options
procmail:LOFILE and LOGABSTRACT settings in procmailrc file