forward mail for a domain with its own certificate

[Evreh Choneh]

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 ? And how I can find documentation on that ?

Regards