Mail server changes may result in an Apple Mail SSL certificate trust warning.
Some email client programs, such as Thunderbird, gmail and kmail, give the ability to permanently accept the warning and continue as normal.
However, Apple Mail does not permit this certificate trust warning to be ignored, or addressed as a valid exception.
Similarly Microsoft Outlook may permit a temporary security exception, but may not permit a permanent exception.
It is possible to workaround this warning IF you trust the new SSL certificate. The error message will give details that can be used to help you decide if you trust the certificate or not. The workaround is to reference the mail server by its hosting configured name, rather than the domain name.
I used a terminal program to get the IP address of the mail server
and then took the IP address to look up its name
An entry such as mail62.example.co.uk was given.
This entry could then be used in place of the website domain in the configuration of the incoming and outgoing mail servers.