[Dovecot] ot: identifying TB IMAP issues

[voytek at sbt.net.au]

I have a user with with W7/TBird/IMAP to my dovecot 2.1.17 server, the
user complains whilst he is composing lengthy emails (with multiple
fowrds/replies quoted in the message) he gets 'hour glass' over compose
windows with some message about not being able to save drafts,

he then powers off ADSL modem, which enables him to continue composing
email, powers on ADSL, and sends email

what if anything can I assess at server end to identify any issues?

any TB tips, what to enable or disable on TB ?

looking at dovecot now (past the issue) with 'doveadm who' I don't see the
user listed

I think the user keeps many messages open concurrently in TB tabs, could
that exhaust connection limit ?

I expect this to be a TB issue? but, hope for some pointers