This error usually occurs when a mail client is using using a secure connection to the mail server under a custom hostname.
For example, this could occur where you have mail.yourdomain.com set as your incoming mail server name with secure settings. This allows you to make a secure IMAP connection, but the SSL certificate installed on our mail servers uses the '*.stackmail.com' hostname - and not 'yourdomain.com'.
It's nothing to worry about, the certificate for '*.stackmail.com' is expected, so you can trust the certificate with the knowledge that the connection will remain encrypted. Once the certificate is trusted you should not receive any further warnings, unless the certificate changes. So in this case, click to accept using the new certificate.
To avoid this warning altogether, or if it keeps popping up annoyingly in your mail software, you can simply change to using the stackmail.com hostnames for your incoming and outgoing mailserver names. Specifically...
Incoming mailserver name:
- imap.stackmail.com (if it's an IMAP account)
- pop3.stackmail.com (if it's a POP3 account)
Outgoing mailserver name:
- smtp.stackmail.com
