How to discard mails sent from a specific local user to external addresses?
To do what OP need, we need a check at transport level, which turn out to be simple.
Add following line to /etc/postfix/main.cf
sender_dependent_default_transport_maps = hash:/etc/postfix/sender_transport_maps
Create /etc/postfix/sender_transport_maps as follow
[email protected] discard
Create postfix map file and restart postfix
cd /etc/postfix postmap sender_transport_maps service postfix restart
This method works because postfix only use transport map for out-bound mail. In this case, instead of using a normal smtp service (smtp:), we use postfix DISCARD service.
The sender_dependent_default_transport_maps didn't work at all for me. Even with smtpd -vv in master.cf to increase debug didn't help let me know why it didn't work (possibly an old postfix version).
Instead I used
smtpd_sender_restrictions = check_sender_access pcre:/etc/postfix/sender_domains, discard
with the sender_domains file containing
/[email protected]/ DISCARD /@domain.com/ OK
for eg, then restart postfix (pcre files do not need to be/can't be postmap'd)