I have debugged this error message for the last couple of days.
Jul 1 23:11:56 lance imapd: LOGIN, email@example.com, ip=[::ffff:AAA.BBB.CCC.DDD], port=, protocol=IMAP
Jul 1 23:11:56 lance imapd: firstname.lastname@example.org: Permission denied
The solution was pretty simple.
The /tmp folder had bad permissions. This server was only meant for hosting email services, so bad permissions on /tmp folder was actually not an issue earlier.
I guess the wrong permissions were caused by my custom XEN node setup using multiple partitions, including a partition just for /tmp.
Debugging was quite hard
Authentication was successful, as I got a different error message when authenticating with a known bad password.
I debugged it using strace. It wasn’t easy, as courier imap forks out child processes for each connection, which I had to strace as well.
# strace /usr/sbin/couriertcpd -address=0 -maxprocs=40 -maxperip=20 -nodnslookup -noidentlookup 143 /usr/lib/courier/courier/imaplogin /usr/bin/imapd Maildir
Connect to port 143 using telnet.
Log in using this command:
i login MY_EMAIL_USERNAME MY_PASSWORD
Then find the imap process PID. Look for a process running as user vmail:
$ ps axuw|grep imapd
vmail 362 0.0 1.0 4616 1344 ? S 01:46 0:00 /usr/bin/imapd /var/spool/mail/vmail/hoven.ws/knut-olav/Maildir/
In this case, the PID is 362. Then attach strace to it using strace -p 362, as sudo.
From the telnet interface, I entered a couple of commands like these:
2 select "INBOX"
5 UID fetch 1:10 (UID RFC822.SIZE FLAGS BODY.PEEK[HEADER.FIELDS (From To Cc Bcc Subject Date Message-ID Priority X-Priority References Newsgroups In-Reply-To Content-Type)])
Then I found this somewhere down into the strace output:
open("/tmp/tmpfWsezjv", O_RDWR|O_CREAT|O_EXCL, 0600) = -1 EACCES (Permission denied)
write(2, "ERR: email@example.com: Permiss"..., 43) = 43
Fixing the problem
chmod 1777 /tmp
As I wrote earier… a simple solution.