Confusing dns issue with a client account imported from cpanel

I recently imported a client account from cpanel. We also moved their domain to a new registrar.

I now host their website and email on my shared hosting system server1.domain.com

Now I am striking a problem that I cannot make sense of...

First...some housekeeping... server1.domain.com is my webhosting/mail server clientdomain.com is the client virtual server name and domain.

So, heres the problem...(well 2 problems actually)

1. If I set the client mx record to server1.domain.com their email fails

However if I set client mx record to mail.clientdomain.com emails start working.

I have another client on the same system (not imported via cpanel) which uses mx record server1.domain.com and their emails are working.

I don't understand how this can be? Is this something the cpanel import has done to the virtual server itself?

Both client accounts have A records pointing at the same server1.domain.com IP address and their websites are functioning perfectly. It's just the mail that is not right.

2. Also, server1.domain.com is using ssl cert domain.com (which is also the first virtual server on it ...ie my hosting business website). When I view ssl certs from postfix, I notice it's displaying as domain.com (and not server1.domain.com).

When i check virtualmin>Server Configuration for my own business virtual server the certificate is for host.domain.com, domain.com, mail.domain.com, www.domain.com.

However, underneath that is says,

This SSL certificate is already being used by : Webmin (domain.com), Usermin (host domain.com), Usermin, Dovecot (host domain.com), Postfix

I am confused by that...why is it saying "host" and domain.com but not server1.domain.com? It seems like webmin and postfix are not using the right certificate...ie one for server1.domain.com.

Is this a problem? (My shared hosting clients mail apps are throwing ssl cert errors for this certificate and I think it's because it's hostdomain.com instead of server1.hostdomain.com)