I guess the error has nothing to do with subdomain as I try with a different domain. Here my bounce mail:
<email@example.com> (expanded from <firstname.lastname@example.org>): host
mail.cayambelibre.com[188.8.131.52] said: 550 5.1.1
<email@example.com>: Recipient address rejected: User unknown in
virtual mailbox table (in reply to RCPT TO command)
So it has to do with virtual mailbox table I guess. Is this a known bug for the version of ISP config I am using?