The certificate does not match the expected identity of the site that it was retrieved from.
The server's identity does not match the identity in the certificate
Hostname in certificate didn't match
The reason for the above errors are due to the fact that the email client is trying to use mail.mydomain.com and the email server is mail.emailserver.com.
So the email client pulls the certificate for mail.emailserver.com and reads that this certificate is for mail.emailserver.com, NOT for mail.mydomain.com. Hence the conflict and it throws an error.
If you can, accepting the certificate should let everything work. It appears that on recent versions of iOS their may be some problems trusting it. The other work around is to use the actual mail server host. So mail.emailserver.com