The error that the other server returned was: 550 550 5.1.1 <montreal@peace-node.net>: Recipient address rejected: User unknown in local recipient table (state 14).
What the other server returns isn’t helpful…we need to know what your server says about it. Specifically, the entries in the maillog when a message is rejected.
There is a section in the documentation about troubleshooting mail problems here:
I suspect you’ve got Postfix misconfigured…you’ve probably set myorigin incorrectly (setting it to just about anything is probably incorrect…it’s not a very friendly option…so leaving it out and letting Postfix decide for itself is a smarter option in 99.9% of cases).