Postfix: local mail only for some users

If you want to allow some users to send "local" mail only, the following postfix configuration will do the trick. With local I mean within the organisation, for which your postfix is configured to receive mail.

In your smtpd_recipient_restrictions, rather at the top of the restriction list, add

        check_sender_access hash:/etc/postfix/access-sender-local_mail_only,

where the file /etc/postfix/access-sender-local_mail_only contains a list of senders which are restricted:

usera@example.com       localmailonly
userb@example.com       localmailonly
userc@example.com       localmailonly

So here we have defined that these users are subject to the restriction class "localmailonly", which we will have to define:

smtpd_restriction_classes = localmailonly
localmailonly = permit_auth_destination,reject

Here we have defined a new restriction class, and have assinged rules to this class. Namely, we only permit authorized destinations (thats the destinations we accept mail for), and otherwise reject the message. In order for this configuration to take effect, we need to convert /etc/postfix/access-sender-local_mail_only to a .db file, and reload postfix (to read the changes to main.cf):

postmap /etc/postfix/access-sender-local_mail_only
service postfix reload