Mail is using wrong outgoing server and From address for replies
I have two acocunts in Mail. One personal (a gmail acocunt), one work (email thru my web host, call it MyHost.com). Used to be, if I replied from a mail sent to my gmail address, the outgoing reply was sent thru gmail's server. Replies to mail sent to me thru MyHost.com went out through the MyHost.com server. Now, no matter what, Mail replies with the gmail address as the sender and the gmail address in the From slot, and it tries to go through gmail's outgoing email sever. This problem started this morning, and has never happened before.
I have changed nothing, and my settings are correct:
In Mail > Preferences > Account Information, the incoming and outgoing servers and addresses are correct for each account. The gmail account has gmail's info, the MyHost.com acocunt has its MyHost.com info. None of this has been modified for several years, and it didn't change a month ago when I upgraded to Mt. Lion. It's still all correct.
In Mail > Preferences > General, the setting for "If outgoing server is unavailable" is (as always) set to "Automatically try sending later" (not "Show a list of alternate servers").
In Mail > Preferences > Composing, "Send new messages from" is set to "Account of selected mailbox"
I have set no rules regarding these things (not sure you even could).
It all worked fine yesterday. Nothing has changed at my end in the last 24 hours.
Is there ANOTHER setting I should be looking for? Is there a preferences/plist file to delete to restore correct behavior of outgoing mail? As it is now, I have to manually select the outgoing server whenever sending from my work account, or replying to an email sent to my work account. This works (sconfirming that my server settings are correct) but it is a huge pain, and I've already forgotten to do it a couple of times, which looks bad.
Mac mini, OS X Mountain Lion (10.8.2), Mail 6.2 (1499)