On our "real" (managed by professionals) server, writing to fake@example.com from Outlook returns an immediate Undeliverable notice, not a bounce message. A PHP script that mails to fake@example.com does not generate a bounce. This is good.

On the local server, writing from Outlook gets a bounce back from MAILER-DAEMON, and so does mailing from the PHP script. This is bad.

If I connect to Postfix by telnet, I get a 550 Reject after RCPT TO: <fake@example.com>. Connecting directly to Amavis on port 10024 does not give a rejection, but all incoming mail goes through port 25, right? (It's the only port open on the firewall, after all.) How do I get rid of the bounces?

I will be happy to provide any additional info, but I'm not sure where to start. Users/Aliases are stored in MySQL.