The las errors had to do with some misconfiguration when i tried to use gid 1000 because that id is used by ftp on my server.
virtual_uid_maps = static:1000
virtual_gid_maps = static:1000
So I am able to use mailaccounts that I alias to some other mail. If I remove alias mails sent to accounts on my server are bounced saying that the user does not exist:
[HTML]<<< 550 <firstname.lastname@example.org>: Recipient address rejected: User unknown in local recipient table
550 5.1.1 <email@example.com>... User unknown
<<< 554 Error: no valid recipients
Reporting-MTA: dns; mail.domain.com