I'm have the same problem with Centos 5.6 Plesk 10.3.0.
In maillog i see:
Jan 24 14:00:26 host2 imapd: IMAP connect from @ [::ffff:127.0.0.1]INFO: LOGIN,
[email protected], ip=[::ffff:127.0.0.1], protocol=IMAP
Jan 24 14:00:26 host2 imapd: 1327406426.717296 DISCONNECTED...