Mac OS X 10.5.1/Mail/ cannot send / MFMessageErrorDomain

I cannot send encrypted/signed messages anymore, even though Keychain
first aid reports no errors.
When sending a message, I get an Alert:
Operation could not be completed, (MFMessageErrorDomain error 1035.)

Sending without encryption and/or signing works.
The appropriate certificate is in the login keychain, and set to trust
always. It has not expired yet.
Sending with my second e-mail address works also encrypting/signing.

It worked until yesterday too.

What could be wrong?

TIA

Marc

--
remove bye and from mercial to get valid e-mail
<http://www.heusser.com>

Marc,

I am having the exact same problem. I saw this on the Apple
Discussion threads:

http://discussions.apple.com/thread.jspa?messageID=6512267

But that was it and I was not willing to nuke my keychain. I did nuke
the certs for the server with the problem but that did not help.

It is interesting to note that I only get this in Apple Mail with one
of my accounts. I have 5 accounts on several different servers and
digitally signing messages works on those other accounts. It is just
this one.

Jonathan