[Dovecot] email stuck in outbox

[barnaby cockcroft]

Can anyone here help me out with a client side Outlook issue? I know
outlook is a horrible imap client :(

We intermittently have a problem where mail gets "stuck" in Outlook
users' outboxes.

Might have anything to do with dovecot? Is there anyway I can
troubleshoot this server side? Are there any settings I can tweak?

We have a new and an old dovecot server (see below) and both exhibit the
same behavior (occasionally) for heavy mail users. 

We haven't ruled out a layer 2 issue but nor have we found one....

If anyone has any ideas (other than suggesting a different email
client), they'd be much appreciated.

Dovecot version(s) 1.19 & 0.99.11

Configuration from 1.19:

# 1.1.9: /usr/local/etc/dovecot.conf
# OS: Linux 2.6.9-78.0.13.ELsmp i686 Red Hat Enterprise Linux ES release
4 (Nahant Update 7) ext3
syslog_facility: local0
ssl_cert_file: /usr/local/etc/ssl/****************
ssl_key_file: /usr/local/etc/ssl/*****************
disable_plaintext_auth: no
login_dir: /usr/local/var/run/dovecot/login
login_executable: /usr/local/libexec/dovecot/imap-login
mail_location: mbox:/home/%u/mail:INBOX=/var/spool/mail/%u
mbox_write_locks: fcntl
auth default:
  debug: yes
  passdb:
    driver: pam
  userdb:
    driver: passwd