From time to time after a migration from cpanel to DirectAdmin, some email clients may generate an invalid certificate warning when retrieving or sending mail. This is more common in Apple-based products (Mac Mail, iphones, iPads), but can affect other email clients as well. To resolve this, change the mail server name from yourdomain.com (or mail.yourdomain.com) to the server name presented in the certificate. All server names in the network will be in the format (server).hmdnsgroup.com. Enter the full server name in the email client settings, then restart the email client. For Apple-based profucts/devices, quit/close the program or application and then reopen it. That should resolve the issue.
If problmes persist, please contact support.