IMO, a real PIA. You'll have to recreate your email account after clearing the email cache. Just deleting the account and recreating doesn't work. I've had to do it twice so far since getting my Droid on 11/6/09.
Here's how to clear the cache:
Settings
Applications
Manage Applicactions
Cilck the menu key
Choose Filter
Choose Running
Choose Email
Click the "Clear Data" button
(WARNING: this will delete all your Outlook/exchange accounts and settings on your phone, but since they're not usable, it doesn't really matter.)
Next time you start the Outlook email program you'll be prompted to setup your account.
A real pain, but not as bad as doing the factory reset and having to setup all the other stuff too. I'd recommend copying all your account settings first if you're not familiar with them.
I hope Motrola/Verizon fix this problem with the next update...
All the above is mentioned in another thread, but I can't seem to find it using the forum search box...