Ok I tried what you said:
- Changed my mail account from mail to mail+forward
- Added onw forward address to the list called "email@example.com"
- Send a mail to "firstname.lastname@example.org"
- Got error (User unknown in virtual mailbox table)
Basically, what you want is ability to send mails to aliases in which case, the mails should be delivered into the mailbox of the mail account for which you have defined them. Problem is that aliases don't always belong to locally hosted domains.. You can have an alias address such as email@example.com ... But I'll check the behavior and see if we can add specific "rules" for aliases that belong to local domains. By local, I mean those for which domains are hosted locally. In reality, those are virtual domains...