How to create email for sub-servers?

I confused on how to do this.

Say I have a top server named:
and I have a sub server named:

When I select in vitualmin and then edit Mail aliases how do I get this so that I can receive emails for my subserver? Or am I going at this wrong?

The case where the above doesn’t work, is if is an “Alias” domain.

Aliases can’t have email accounts, only Virtual Servers and Sub-Servers can have email accounts.