removing malicious certificate that is blocking mail

What makes you think it is malicious? I would expect it to be a normal part of the chin of trust used to encrypt communications across the internet. Seriously I am asking this question because I really doubt a certificate issued by DigiCert would be malicious in this instance, especially as they also provide the encryption certificate for the Centurylink.net website. So they are the chosen provider for century link, or lumen technologies as they are now known, It is probably not "stored" locally as the server will offer it when you attempt to connect and based on the issuing authority it will be automatically trusted by Thunderbird based on a chain of trust where digicert is a certifying authority. I think you might need to actually explain what the issue is because I doubt the certificate is the real issue.