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I had big issues with timeouts until I realize the the imap and mbox format isnt a good choice if your users have huge naiboxes. At my server there were users with mailboxes arount 700MB to over 1GB. each time one of those users checked their email the imap server had to open this huge file and parse it, causing I/O issues.

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The only solution I found it was switch from mbox format to maildir format.

I am changing the subject line to reflect the discussion.

I too have a similar case with large mailboxes. My user mix is 70% POP3 vs. 30% IMAP. Would maildir still be the best recommended format? What is the opinion of the experts here?

The reason I am asking is that I am preparing to upgrade my mail server to RHEL 5.3. That would mean my Dovecot will be atleast 1.0.7, which should not be bad. If the recommendation is that I go from mbox format to Maildir, I want to plan that as well. However I don't want to land up in a situation where, I have to move back from Maildir to mbox again. So please do advice me on the best.

BTW one question: Assuming that my server has enough resources, ( 8 CPU @ 3.0GHz, 8GB RAM, 2Gbps FC Storage directly attached to the server & Gigabit NIC) what is the maximum number of concurrent connections that Dovecot can handle POP3 + IMAP combined? And I am asking for all the processes combined:

POP3-login processes IMAP-login processes POP3 sessions IMAP sessions

Any TCP/IP kernel parameters to be tuned? or maybe in dovecot.conf?

Any advice will be immensely valuable to me while doing the migration.

Thanks and Regards V S Rao