forward mail for a domain with its own certificate

Hello I want to do the following : I've a domain A for all my users. For some users I have a domain B and I would like to do this : a) A user from domain B can send a mail only for another user of B b) All mail from B use the certificate of domain B to crypt and sign mail Can Postfix do that ?

Prevent sending message with known user out of LAN

Hello, How to prevent Postfix to send an email with a known user from outside my LAN if he is not authenticated ? I have the following parameters but a user can still do it : smtpd_helo_restrictions = permit_mynetworks, permit_sasl_authenticated, #check_helo_access hash:/usr/local/