Found out where that message came from - when I try to login to the webmail on my host server. Either the original account or one of the remote imap ones gives this:
Secure Connection Failed
webmail-srv2.servage.net uses an invalid security certificate.
The certificate expired on 20/08/09 08:49.
(Error code: sec_error_expired_certificate)
* This could be a problem with the server's configuration, or it could be someone trying to impersonate the server.
* If you have connected to this server successfully in the past, the error may be temporary, and you can try again later.
However, both Thunderbird and Claws Mail connect OK, so I have no idea what is going on. If I do not get to the bottom of it I shall contact servage.