I have setup Mail in a box for one domain and everything is working fine except some users are going to use Gmail to send and receive these emails. so what i have done is i put the alias or forwarder to this users which forwards the email to their Gmail account and we added the firstname.lastname@example.org to gmail send as settings.
The problem is while the aliases are configured on mail in a box when we end out emails using gmail we get the below error message
553 5.7.1 <email@example.com>: Sender address rejected: not owned by user [firstname.lastname@example.org)
but as soon as i remove the aliases send as starts to work, i have also tried using roundcube to forward the emails but i get the message that the gmail is not in the virtual alias table.
any help is appropriated