Solved an email problem tonight which has been driving me nuts, documenting it here so the Google spiders can get this solution to other sufferers.

Symptoms: Mac OS X Mail.app takes a very long time to download new messages, and while a download is occurring, windows become temporarily unresponsive to clicks and keystrokes. (E.g., clicking another account takes a minute or longer before it appears in the window.) Download time is increased, apparently geometrically, when multiple accounts are checked or when a single account has many messages waiting. Activity window seems to spend a lot of time on “routing messages” and “processing mail rules” even when there are no rules to process.

Useless fixes: the usual cure-all for a Mail.app slowdown, rebuilding the database through message import by deleting the Envelope Index, doesn’t do diddly. Likewise the more drastic solution of moving ~/Library/Mail and reimporting the entire kaboodle. Ditto deleting all of the mail rules. Ditto deleting all of the plug-ins from the Mail library Bundles folder.

Actual fix: removing GrowlMail.mailbundle and ProxiMail.mailbundle from /Library/Mail/Bundles. Yes, Virginia, you have a system-level Mail library, and plug-ins there will affect all users on your system, despite being off in a dusty corner of your Mac OS where angels fear to tread. (I’m very glad I didn’t bother trying a mail import from a new account.)

In fairness, both of these bundles will Attempt to Do Something upon the receipt of any incoming message, so I have no idea which of these is the culprit. The other is probably blameless. Or maybe they’ve been beating up on each other and that’s the problem. Someday I will attempt to determine what went wrong here; in the meantime I’ll rest on the laurels of a bug well stomped.