I use the WHH secure servers for checking and sending e-mail. Recently, when trying to send e-mail, I receive an error message prompting me to create a security exception. (This is new in the past few days.)

The error declares "This site attempts to identify itself with invalid information," and that the "Certificate is not trusted, because it hasn't been verified by a recognized authority." From searching around online, my best interpretation is that the root certificate is not properly identified and so the rest of the chain is considered suspect. (This is what checking at ssl-tools.net suggests, e.g.)

So, what's going on? Do I need to create this security exception / is that a good idea?

I use Postbox (postbox-inc.com) e-mail, which was based on Thunderbird originally; I have read advisories for Thunderbird that seem similar to this issue, but I can't tell for certain if they are the same thing or not.