Can you provide a bit of the information? :) If you want to change hostname, you can edit it in /etc/motd or at your Cpanel/ISPmanager/etc.
Try Easy Burn ISO - it's freeware
Run please this command:
dig mx mycompany.com
You'll be surprised I think ;) Why emails stoped coming - this is the second thing.... you can...
I think you also need to modify/check postfix-config:
- backupmx = '1' and relay_domains = [your domain]
Separate names with a comma.