Can't figure out how to use subdomain for mail.

[SkIRmiS]

Hello,
The system is great and everything is working well now. But can't understand how to use subdomain for mail, example user@test.mydomain.tld. When I'm creating a new user, I can use only @domain.tld part. Is any chance to change something a little bit, what I can choose subdomain. I tried to use aliases. I want what only subdomain will work 4 user at moment.

Thanks
SkIRmiS