Getting a “mail certificate name mismatch” / “cannot verify server identity” error?

This error usually occurs when a mail client is using a secure connection to the mail server under a domain that hasn't been covered by our SSL certificate.

For example, if imap.yourdomain.com is being used to set up mail in a mail client but you do not have an SSL certificate, this will result in the ”mail certificate name mismatch” / “cannot verify server identity” warning.