Funny how you have asked the supposed experts, the ones offering the certificate and they have sent you to the free support of something else because they just could not be bothered explaining it. Or is it they are simply not capable. A rather worrying consideration given their status a "professionals" You are attempting to contact a mail server named mail.myfairpoint.net Your provider of that mail server it offering a certificate for a server with the domain name carrierzone.com So between you and the people that run the server you need to decide if you think the owners of the certificate and your mail server are to be considered one any the same or not. If not ask your provider to fix the certificates so you do not get a prompt. This will cost several hundred dollars in most instances. It is a feature of cheap domain hosting that appropriate certificates are not used. A case of you get what you pay for. The port used 143 would indicate that there should be no connection security used, but you are using SSL/TLS perhaps you just need to set the connection security to none. So to amuse myself I ran a test on the mail.fairpoint.com mail server https://www.immuniweb.com/ssl/?id=cyWBnsmW The F they get for security might be an indication of where the issue lies. It might also be in the fact carrier zone, whomever they are do not recommend the use of SSL/TLS http://carrierzone.com/Thunderbird.htm I did try and check the setting for your server from your provider, but in typical US ISP form, they do not publish the information for non customers so I guess you might want to ask them what server settings they recommend for users of mail applications. They are the ones running the service and are the only ones that actually know what setting you should be using.