Postfix MX – Configure Postfix to Use External MX Servers for Delivery of Local Mail if User is Unknown

The remote server behave according to SMTP specification RFC 2821 especially in section 4.2.1. Because you reject with code 450 then remote server will retry to send it until the queue expired. Quote from the RFC

4yz Transient Negative Completion reply

The command was not accepted, and the requested action did not occur. However, the error condition is temporary and the action may be requested again. The sender should return to the beginning of the command sequence (if any). It is difficult to assign a meaning to "transient" when two different sites (receiver- and sender-SMTP agents) must agree on the interpretation. Each reply in this category might have a different time value, but the SMTP client is encouraged to try again. A rule of thumb to determine whether a reply fits into the 4yz or the 5yz category (see below) is that replies are 4yz if they can be successful if repeated without any change in command form or in properties of the sender or receiver (that is, the command is repeated identically and the receiver does not put up a new implementation.)

Now the problem is your server was configured so it gives reply code 450 temporary reject instead 550 permanent reject.

Since you haven't share postfix configuration, then we can't pinpoint which config that caused that behavior. You can start examine these parameters below and checks that that parameter show in output of postconf -n.

• unknown_local_recipient_reject_code
• soft_bounce
• defer_if_reject

As you already have example.com as a local domain configured in either mydestination or virtual_alias_domains you can't achieve this by also adding it to relay_domains.

Instead, you should add a transport(5) map that can override delivery rules defined by the default transport:nexthop used by mydestination, virtual_alias_domains etc.

DESCRIPTION

The optional transport(5) table specifies a mapping from email addresses to message delivery transports and next-hop destinations. Message delivery transports such as local or smtp are defined in the master.cf file, and next-hop destinations are typically hosts or domain names. The table is searched by the trivial-rewrite(8) daemon.

This mapping overrides the default transport:nexthop selection that is built into Postfix.

Add transport_maps to your /etc/postfix/main.cf:

transport_maps = hash:/etc/postfix/transport

And then add to /etc/postfix/transport:

xyz@example.com    smtp:example-com.mail.protection.outlook.com
@example.com       :
*                  :

(Remember to postmap /etc/postfix/transport as you are using hash, Berkeley DB lookup table.)

This will use default transport rules for all @example.com and anything else (*) leaving other delivery configuration as is, but forward all email to xyz@example.com to Office 365 via SMTP: the default pattern for Office 365 MX record for companies is <domain>.mail.protection.outlook.com.

Using transport_maps is a global solution that will work with any external SMTP server. In a specific scenario, where a) the external account is on Office 365 and b) domain is configured on Postfix virtual_alias_domains it could also be possible to add a virtual alias redirecting all mail to Office 365 user's initial domain (tenant) address, xyz@example.com xyz@contoso.onmicrosoft.com.