Thunderbird won't accept a certificate when making a POP3 connection over SSL, claiming it is invalid because it "contains the same serial number as another certificate issued by the certificate authority. Please get a new certificate containing a unique serial number."
There's no option to ignore the error and use it anyway, but then again, it's nothing to do with the server. This is Thunderbird's coy way of telling you that there are some expired certificates that should be removed.
