[Users] Extreme slowness with Exchange 2013 and IMAP, SOLVED (ish)

Steve Chupack steve.chupack at dealer.com
Mon Aug 26 01:13:11 CEST 2013

´╗┐My organization recently migrated to Exchange 2013 from Exchange 2010.

I am not posting this to start a pro/con discussion of Exchange, just to help the handful of people who might be (or soon will be) using *any* IMAP client with Exchange, not just Claws.

After switching to 2013, IMAP became about 10x slower than usual (basically unusable). This was actually the case with *all* clients, although was perhaps slightly more apparent with Claws because it's actually possible to watch the claws log file as every IMAP command is issued, and two, to be honest, Claws is a little hampered by its less than 100% multithreaded nature when scanning folders/mailboxes.

A colleague discovered two new settings in Exchange that seem to severely throttle IMAP operations.

New IMAP throttling metrics introduced with E2013 were limited re-indexing procedures. Set the following to unlimited:

ImapMaxBurst                              : 60000
ImapRechargeRate                          : 240000

This helped a lot, although I'd saw it still takes approximately 2x longer to scan/enumerate folders. Again, this is an issue/change with Exchange, not a Claws problem. Although if I could change one thing about Claws, it would the ability for the app to do more things while enumerating messages and scanning folders. I believe there was a thread about this a while back and I believe Colin or one of the other Devs indicated that true multithreading would require a full rewrite. Someone please disabuse me of that notion of I'm wrong.

Hope this helps the 12 people in the world who might be using Claws/IMAP with Exchange 2013. Luckily I work with an Exchange admin who's willing to look into issues reported by the small number of people who either can't or prefer not to use Outlook. ;-)  


Steve Chupack | IT Systems Administrator
V: 877.327.8422 x 1242
Steve.Chupack at dealer.com | www.dealer.com

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