554 5.7.1 <user@example.com>: Relay access denied

Common causes

• The mail client did not authenticate (SMTP AUTH) before sending to an outside address.
• The client's IP is not in mynetworks.
• On an inbound server: the recipient domain is missing from mydestination, virtual_mailbox_domains or relay_domains, or MX points at the wrong server.

How to fix it

1. Enable SMTP authentication in the mail client and send on port 587.
2. On the server, check smtpd_relay_restrictions includes permit_sasl_authenticated and permit_mynetworks before reject_unauth_destination.
3. For inbound mail, add the domain to the right Postfix domain list and check the MX record.