Thunderbird doesn't trust MS Live POP server?

Quite surprisingly, I was not even thinking about writing an article when I set up my Thunderbird for a MS Live mail account. 

One day, without any notice, I had the following warning:

Because I use a 64 bits version (Shredder), there is no locale version of if, ie. in French. So I wonder what a lambda user is supposed to understand about this warning...

Let's have a look at the certificate by itself:

It appears that the POP3 Server of MS Live Mail tries to present a certificate for pophm.sympatico.ca. Where, yes, it seems to be a problem there... and Shredder is right warning me about it.

Once again, how a lambda user is supposed to handle that? How could he or she make the difference between that case (which could certainly be accepted as an exception), and a real fraud/phishing attempt?

 And the most surprising part could be that MS Live Mail client does not prompt any warning while retrieving the same MS Live Mail account... does it hide a security weakness?