This error usually happens when a mail client is using a protected connection with the mail server under a custom hostname.
For instance, this could happen where you have imap.yourdomain.com indicating imap.stackmail.com. This enables you to make a safe IMAP connection, however the SSL authentication introduced on our mail servers uses the '*.stackmail.com' hostname - and not 'yourdomain.com'.
It is nothing to worry about, the certificate for '*.stackmail.com' is normal so you can confide in the declaration with the learning that the connection will remain encoded. Once the ceritifcate is believed you should not get any further warnings, except if the certificate changes.
To avoid the warning altogether, you could change the mail customer to utilize the stackmail.com hostnames. These are: imap.stackmail.com, pop.stackmail.com or smtp.stackmail.com for IMAP, POP or SMTP respectively.
