You can set whatever you want… Just follow the path to the settings and set it up from there…
mx.mydomain1.com
f-up.mydomain2.com
liam.mydomain3.com
Before you create a new server, you would need to go back into the settings and change the name again. Not sure if it would affect the servers you created so far. I never had this use-case and probably never will. So give it a go and try yourself.