I take back my original answer. Do not contact mail.com, their certificate is updated. You are seeing the old legitimate but expired certificate. Accept the new certificate. You might need to delete and recreate the configuration in your mail program (sometimes that happens with Outlook or some phones) to force them to download the new certificate.
I see this error in batches, not for every pop request I made.
I think mail.com using some kind of round robin service based on multiple servers/machines. And one/few of those boxes was configured improperly (with expired cert file). This is the only explanation I can suggest based on irregular error pattern.