mail app certificate - debug-pop3.live.com

what's this all about? I get a request to say that debug-pop3.live.com has a certificate that is not valid and it could be someone posing as the real one.

i choose cancel, but what can i do to make sure that it is the real server?

I have searched google but found nothing to shed light on whether to trust it or not.

Thoughts people would be nice 🙂

Daz