These filenames mey be related to procmail temporary files created during delivery:
Try an strace (or equivalent) like this:
$ strace procmail &1|grep O_EXCL open("/var/mu/apap/Maildir/_YgG.ZvMvEB.hyperion", O_WRONLY|O_CREAT|O_EXCL, 0444) = 6 ...
Are you using procmail? If yes, something may be breaking it.
It seems the culprit may be procmail which breaks. But only when I am logged on my imap account. When I am logged on imap, a verbose procmail gives: $ procmail &1 procmail: Couldn't create "/var/mail/n" procmail: [5957] Tue Jul 18 16:04:16 2006 procmail: Assigning "LOCKFILE=/home/n/procmail.lock" procmail: Locking "/home/n/procmail.lock" procmail: No match on "^Return-path:.*exim-users-bounces.*" procmail: No match on "^Return-path:.*dovecot-bounces.*" procmail: Couldn't create or rename temp file "/home/n/Maildir/new/msg.eW3"
When not logged into my imap account, it all works well: $ procmail &1 procmail: Couldn't create "/var/mail/n" procmail: [3092] Tue Jul 18 16:06:03 2006 procmail: Assigning "LOCKFILE=/home/n/procmail.lock" procmail: Locking "/home/n/procmail.lock" procmail: No match on "^Return-path:.*exim-users-bounces.*" procmail: No match on "^Return-path:.*dovecot-bounces.*" procmail: Assigning "LASTFOLDER=/home/n/Maildir/new/msg.R33" procmail: Opening "/home/n/Maildir/new/msg.R33" procmail: Acquiring kernel-lock procmail: Notified comsat: "n@0:/home/n/Maildir/new/msg.R33" Folder: /home/n/Maildir/new/msg.R33 1 procmail: Unlocking "/home/n/procmail.lock"
Note that I used a static lock to test. But it seems that dovecot is doing something with the file /home/n/Maildir/new/msg.R33 very likely moving, deleting or renaming it as I cannot find it back afterwards.