Dovecot-2.0.3 reported:
Error: Maildir: Found unwanted directory /path/to/mail/user/cur/:2,FST, but rmdir() failed: Directory not empty
and sure enough, this directory really does exist and contain a valid message file:
# cd /path/to/mail/user/cur/ # ls -lR total 384 drwx------ 8 vmail vmail 272 Sep 24 09:06 . drwx------ 13 vmail vmail 442 Sep 24 09:06 .. -rw------- 5 vmail vmail 41045 Sep 24 08:44 1285336100.M879758P1166.my.mail.server,S=41045,W=41755:2,RST -rw------- 5 vmail vmail 41045 Sep 24 08:44 1285336100.M910424P1166.my.mail.server,S=41045,W=41755:2,RST -rw------- 5 vmail vmail 41045 Sep 24 08:44 1285336337.M656599P1166.my.mail.server,S=41045,W=41755:2,ST -rw------- 5 vmail vmail 41045 Sep 24 08:44 1285336355.M88818P1164.my.mail.server,S=41045,W=41755:2,FS -rw------- 1 vmail vmail 14953 Sep 24 09:00 1285336838.M483116P5720.my.mail.server,S=14953,W=15225:2, drwx------ 3 vmail vmail 102 Sep 24 09:00 :2,FST
./:2,FST: total 88 drwx------ 3 vmail vmail 102 Sep 24 09:00 . drwx------ 8 vmail vmail 272 Sep 24 09:06 .. -rw------- 5 vmail vmail 41045 Sep 24 08:44 1285335855.M242128P1165.my.mail.server,S=41045,W=41755 #
Dovecot must have created this non-Maildir++-ish directory, but why, why in cur/ not tmp/, and why didn't it clean it up? (Not the "Directory not empty" error specifically; I mean in the larger sense.)