Using dovecot pre-authenticated besides another IMAPd

Goal: Test Dovecot with Maildir style mailboxes under the same environment and circumstances as the current IMAPd (UW IMAP with MBX style mailboxes), but without modifying the current setup.
Method: Use pre-authenticated IMAP and ssh as secure tunnel. MUA supporting this method are e.g. mutt, pine and alpine.

What to change

If dmail or tmail are used from inside shell scripts to deliver mail, replace them by Dovecot's MDA (called LDA and residing in /usr/lib/dovecot/deliver in Debian). You need to use the -d option and add the user name since the setuid delivery still has EUID=0. In our case this was necessary in /opt/bin/spamdeliver.sh. (maildrop doesn't work very good here since it needs a filter rules file belonging to the recepient of the mail instead of a simple command line option.)
If dmail or tmail are used in the .procmailrc, replace them by the Maildir folder itself. Don't forget the trailing slash to tell procmail that this mailbox is a Maildir style folder.
.muttrc: Replace set tunnel=“ssh -q imap.phys.ethz.ch /etc/rimapd” with set tunnel=“ssh -q imap.phys.ethz.ch /usr/sbin/dovecot –exec-mail imap”. Calling /usr/lib/dovecot/imap directly will ignore the dovecot configuration.
Set the DEFAULT and MAILDIR variables in .procmailrc to the right Maildir style INBOX and folders directories.

Dovecot has an mbox_snarf plugin to mimic UW IMAPd's snarfing of incoming mail from /var/mail/$USER to e.g. /var/imap/$USER/INBOX, but this only works if both are in mbox format and not with a Maildir style mailbox as snarf destination. So we deliver directly into the Maildir style INBOX using procmail.
The snarfing allowed to easily identify unused mail accounts by setting a per-user quota on /var/mail. With Maildir this can be replaced by checking the size of e.g. /var/imap/$USER/new/ with du.