Email issue: You have received an invalid certificate...

I’ve been using Interworx for around 3 months now. My email has been working fine the entire time, until today. Today when I attempted to send an email I received the following message:

You have received an invalid certificate. Please contact the server administrator or email correspondent and give them the following information: Your certificate contains the same serial number as another certificate issued by the certificate authority. Please get a new certificate containing a unique serial number.
That message was from within Thunderbird, Mozilla’s email software. Apparently, for some reason deleting some certificates from within Thunderbird cleared up this issue, but brought to light a different issue: Who is That is the domain which comes up as the ‘Issued By’ ‘Common Name’ in the default email security certificate. would be understandable, but who is / how are they associated with Interworx? I was just very surprised to see that domain name on the default interworx email cert. I’m not sure if it was the same CN previously.