Mail server hostname issue

[Karabaja]

I've ordered a new server with same hosting company and they still had old one on the record. So when choosing the hostname I couldn't choose the one I wanted as it gave me an error that domain is already in use.
For example I wanted to use my.domain.com but had to use my.domain1.com instead.
After server was ready I've changed the hostname in WHM to correct one.
Mail server for my domain is set to my.domain.com but somehow cPanel still has my.domain1.com as mail server when I go to email accounts, configure mail client page.
Even though when I go to mx entry in cPanel it shows correctly my.domain.com.

Any idea please where could cPanel be pulling old hostname from?

Hosting support said old hostname was still there along with new one in /etc/hosts but they've deleted it and issue is still there.

I've contacted them again but just in case they don't resolve it I was hoping I could get some help here.

[Karabaja]

Never mind, sorry.
I've got a reply from them. Something to do with switch to dovecot, ssl etc.
There is still some issues to sort but I think they'll take care of it.

[Spiral]

There is a few DNS things and few files to edit in /etc when switching hostnames to get everything updated properly on your end.

You also should contact your provider and have them update the reverse IP
pointer in their systems to reverse resolve (PTR) to your new hostname so
that mail will be both accepted at other hosts and routed back properly.