Messages in Mail have no "From" field

Question

Weird little thing.
I had an account set up as IMAP up until recently, when Exchange became available. So I switched to an Exchange (2010) connection as natively supported.
A month later, I'm just now noticing something very, very strange: when configured as Exchange, messages I send through that account are being saved in the appropriate Sent folder, but the headers don't seem to contain a From: field. Messages sent through OWA on the same account have From: properly written into the header.
I'm looking at long headers for both, and the OWA-sent messages appear correctly. The ones sent through Apple Mail have the subject, date, to, and the usual other fields -- including X-Mailer: Apple Mail -- but there is no From: field.
Was trying to import some sent mail into Gmail for safekeeping and noticed a whole bunch of "(unknown)" appearing in the from/to column.
Anyone have any idea on how I can fix this?

Answer 1

The following test might be helpful in deciding what to try next: Create a New User Account, and as that New User try to set up the Exchange account. If not familiar with setting up a New User Account, see:
http://docs.info.apple.com/article.html?path=Mac/10.5/en/8235.html
This will be a useful test to now check this behavior in another User Account, and of course is not meant to suggest a permanent switch to the New User Account, but rather a test of the Mail application outside of your normal User Account and its existing setup data.
If all works as it should in the New User account, then in the original user account I would next try removing the com.apple.mail.plist file at Home/Library/Preferences, after quitting Mail -- could remove both accounts before doing or not, since as Exchange or IMAP (Gmail?) no messages would be lost. Relaunch Mail, and if you did not remove the account, simply re-enter the account set data. If you didn't remove them, Decline to Import anything even if prompted.
Ernie