Apple has changed the way that Mail.app interacts with plugins in Snow Leopard.
As Ken Aspelagh describes it 
on the Mac Observer:
“Each new version of Mail and the associated Message framework includes a unique code. Plug-ins have to explicitly declare themselves compatible with each new version,” Mr. Aspeslagh told The Mac Observer. “There is some sense to this as plug-ins are closely tied to the inner working of Mail, and stand the chance of breaking when Mail.app updates. Apple didn’t want to take this chance, so now they automatically disable all plug-ins with each update.”
So today’s 10.6.2 update is likely to break some plugins.
Most developers are only too aware of this issue, and will get updated plugins out fast.
Greg Welch, who develops the MailRecent and MailFollowUp plugins, has already got new, 10.6.2-compatible versions out the door.
Eric Hinterbichler has done the same with his new Herald Notification utility.
Other developers are sure to follow quickly. Some acted weeks before the update appeared, like Aaron Harnly who pushed out a compatible version of his Letterbox plugin three weeks ago.
10.6.2 broke the Mail clipping plugin from Omnifocus, but fortunately this can be restored simply by reinstalling it.
Other plugins, especially those not so actively or quickly supported, might be more difficult.
langui.sh has published some Terminal commands that will (potentially) revive other busted plugins. He has written them for GrowlMail but they could easily be adapted for other plugins, and “trick” them into working again.
It’s worth a shot, especially if you are in love with a plugin that’s not so well supported. However, maybe make a backup of the plugin’s bundle first. You know, just in case.
Can I add Growl Mail plug-in to the casualties of 10.6.2
As far as the plugin for DEVONthink Pro Office is concerned: just reinstall it again and it will work.
Open the bundle contents, edit the Info.plist, and add to the SupportedPluginCompatibilityUUID, the last two:
225E0A48-2CDB-44A6-8D99-A9BB8AF6BA04
B3F3FC72-315D-4323-BE85-7AB76090224D
2610F061-32C6-4C6B-B90A-7A3102F9B9C8
99BB3782-6C16-4C6F-B910-25ED1C1CB38B
2F0CF6F9-35BA-4812-9CB2-155C0FDB9B0F
0CB5F2A0-A173-4809-86E3-9317261F1745
Works with GrowlMail, Letterbox, and Mail Attachments Iconizer.
Update for Mail.Appetaize is out. Here: http://bit.ly/uI9Vd
Letterbox also got busted, but the BETA works fine. :)
http://harnly.net/2009/software/letterbox/letterbox-beta-5-for-snow-leopard/#more-240
SigPro is broken as well :-(( my favourite plugin for Mail as it palyed my Itunes as a sig.
UPDATE: a fix released .. thx
http://www.littleknownsoftware.com/2009/11/signatureprofiler_153_released.php
After installing Snow Leopard every time I get an automatic update I lose the ability to send email. I have both Outlook Express &Mac installed but neither of them work. So frustrating since I am such a novice that I don’t know what I do, if in fact I do anything, to reverse this. Can anyone help me to fix & understand this annoying problem?