Email only servers

I have run Virtualmin for some years hosting numerous small to medium websites.
Recently I had a request to host email only for a client whose website is being hosted elsewhere but they don’t do emails. (All to do with industry site he is tied to etc.)
Anyway I setup a simple virtual server and the single email address he needs and the website host adjusted the mx entries to point to my ip etc. - All good, and seems to work okay.
Now I have a second request by the same person to host 2 more email addresses, but these are from a different domain.
He requires that all email sent from the various email addresses actually show the appropriate domain etc.
Do I need to setup yet another virtual host for this domain as well or is there a way I can host multiple domain emails on a single virtual domain.
IE if I have and and, can I setup a single domain like and host emails like, and etc.?
OR do I need to setup a seperate domain for each?
The important thing is that replies etc have to appear to come from the appropriate domain etc.

I do the same for a number of mates of mine… just set up their domain like normal and ask them to change the MX record to point to your server. Your server should then handle the incoming email message.