Cannot create working alias with email

Ubuntu 16.0X

I did this before for another domain on the same machine with no issues.

I am trying to create an alias of a domain with email. It appears to work. The SSL is regenerated from LetsEncrypt. I create one user. I can send an email from that user, but I cannot receive email. I either get a “loops back on myself” error, or this:

An error occurred while sending mail. The mail server responded:
5.1.1 test@domain.com: Recipient address rejected: User unknown in virtual alias table.
Please check the message recipient “test@domain.com” and try again.

The last time I did this, I think the virtual domains were auto-created in postfix, but in this case, nothing was. Is there a way to auto-generate the records? if not, what records are needed? What am I missing in how to create this type of domain/account?