(Just to be fair, I think the SSL cert business is a scam, so there.)
I transferred some domains to NameCheap, and they were selling a $2 or $3 SSL cert as a kind of almost-freebie to registrants. So I bought it. The bad news? The certificate authority is not in the Mozilla Thunderbird email client's list of trusted authorities. So Tbird users will get the annoying warning dialog box.
Someone on NameCheap's forums complained, and then the tech support person said something to the effect that "well, that's out of our hands". They could have warned that Tbird didn't have the cert installed. This would have been a hassle for someone running a LAN full of Tbird users, you know? Because certs for mail servers are becoming more of a necessity than for web servers -- the data in email is more sensitive, and email is being outsourced to external providers more. For the past year or longer, Gmail has been delaying mail to sites with self-signed certs.
BTW, the name of the cert is "PositiveSSL CA 2" and it's sold by Comodo. (Yeah, that Comodo that got socially engineered.)
Looking at Firefox, it doesn't seem to have it. It has a similar named one "PositiveSSL CA", but not the "2". So I installed the cert onto our website, and it worked. Not sure how, but it did.
Now, I'm just hoping that Gmail accepts the cert for the STARTTLS feature in our email server.