"David.M.Clark" <david@davrom.com> writes:

I do have some customers using Outlook or Windows Live Mail, and these are for the most part working fine with IMAP - I don't do POP. ... The issue starts when you add an IMAP user to the Outlook client and upon opening it, initially, it tries to find a "Sent" Items folder under IMAP to send from. To this end I have traditionally gone into the dialogue box that opens, click on the IMAP account to receive a list of possible folders, but it now immediately crashes indicating Outlook has had an error, and it all goes to hell from there. Outlook then cannot open but keeps crashing with its "unknown error". Up to last week at another site with Windows Live Mail, this was not an issue.

Just a shot in the dark, but have you enabled the workarounds?

http://wiki2.dovecot.org/Clients

Some clients like, like Outlook, have known problems which dovecot can dance around. in 20-imap.conf

# Workarounds for various client bugs:
#   delay-newmail:
#     Send EXISTS/RECENT new mail notifications only when replying to NOOP
#     and CHECK commands. Some clients ignore them otherwise, for example OSX
#     Mail (<v2.1). Outlook Express breaks more badly though, without this it
#     may show user "Message no longer in server" errors. Note that OE6 still
#     breaks even with this workaround if synchronization is set to
#     "Headers Only".
#   tb-extra-mailbox-sep:
#     Thunderbird gets somehow confused with LAYOUT=fs (mbox and dbox) and
#     adds extra '/' suffixes to mailbox names. This option causes Dovecot to
#     ignore the extra '/' instead of treating it as invalid mailbox name.
#   tb-lsub-flags:
#     Show \Noselect flags for LSUB replies with LAYOUT=fs (e.g. mbox).
#     This makes Thunderbird realize they aren't selectable and show them
#     greyed out, instead of only later giving "not selectable" popup error.
#
# The list is space-separated.
imap_client_workarounds = delay-newmail

Joseph Tam <jtam.home@gmail.com>