Who is your email provider?

You need to inspect the certificate to see what server name (etc) it declares. It is possible that the operator has set it up improperly so that the actual name and the certificate name are different.

Ideally, you inform the email provider and they fix it. If they cannot or will not, then I'd expect it to work if you set it as a permanent exception, having convinced yourself it is genuinely the server you expect to be using.

I have had to do this myself with a small provider whose certificate and server didn't match. I guess I might have been their only user at that time choosing to use SSL.

Thanks for your help Chosen Solution - it seems to have righted itself, not sure how it did it but it's working for now