Certificate for server is Invalid ??

Hi,
Never had a problem with my mail before, but suddenly seems to have completely screwed up. Every time i try to send something it says



Mail was unable to verify the identity of this server, which has a certificate issued to “ ______”. The error was:

The certficate for the server is invalid.

You might be connecting to a computer that is pretending to be “smtp. ___.com”, and putting your confidential information at risk.

You can try to send using a different server. All messages will use this server until you quit or change your network settings.

I know nothings been changed on the server and I haven't changed anything in mail. It's still working fine on my other mac pro, which has exactly the same settings.
Any ideas? it really ***** not being able to send mail.
Many thanks & have a great day =)