SMTP error 5.7.2 Mailing list expansion prohibited

What SMTP error 5.7.2 means

You are allowed to email the organisation but not that particular list. A list manager checked who you were before expanding the membership, found you were not on the permitted senders, and stopped there with 550 5.7.2. Because no expansion took place, not one member received a copy. It is genuinely uncommon on Microsoft 365, which usually answers the identical situation with 5.7.1, so most real sightings come from Mailman, Sympa, Google Groups or a bespoke list running on somebody's own server.

Security or policy status
Sent by The list manager, or the mail system hosting the mailing list
Rare

What SMTP error 5.7.2 Mailing list expansion prohibited actually means

A mailing list address behaves differently from a mailbox. Instead of storing the message it looks up a membership and generates a copy per member, and most list software applies a posting policy before doing so. This code is that policy declining. The comparison worth drawing is with 4.2.4, which describes a list that could not be expanded — a genuine fault in the expansion, temporary, and worth retrying. This one is a list that would not be expanded, which is a decision and will not change on retry. Note also that the address you are judged on is the envelope sender, which is not always the address shown in the From line of your client.

The sender is not authorized to send a message to the intended mailing list. This is useful only as a permanent error.
— RFC 3463

How 5.7.2 appears in a bounce

A server reporting this condition sends it alongside a three-digit reply code, and a bounce prints the two together. The registry lists 550 5.7.2 as the pairing for this status.

The leading 5 is the server's verdict rather than part of the code's identity: it marks this as a permanent failure, which means the sending server has given up and returned the message.

What causes SMTP error 5.7.2 on a business phone system

A subscribers-only posting policy and a second addressTrade association and user group lists commonly accept posts only from subscribed addresses. Somebody who joined with their work address and then replies from a personal account, or from an alias their phone uses by default, is refused even though they are a member.
A moderated list that rejects rather than holdsWhere moderation is configured to bounce non-member posts instead of queueing them for a moderator, there is no human in the loop to approve anything. The sender sees a refusal and the list owner never learns the message existed.
A Google Group set to members-only postingGroups created inside a Workspace tenant default to restricting who may post. When the same address is later published as a public contact point, the restriction is rarely revisited and outside enquiries silently fail.
A forwarder rewriting the sender before the list sees itIf your mail reaches the list via a forwarding address, the forwarder may rewrite the envelope sender to its own domain so that SPF still passes. The list then compares that rewritten address against its membership, finds no match and refuses.

How to fix SMTP error 5.7.2

Confirm which address the list actually sawLook at the Return-Path in the bounce rather than the From address you typed. Those two differ more often than people expect, especially on phones and on anything sending through a third-party platform.
Have the owner permit the address rather than work around itEvery list manager has an allowed senders concept. Adding the one address you actually use is a two-minute change for the owner and avoids everybody involved maintaining a second mailbox purely to post.
Suggest moderation instead of rejection for non-membersA list that holds unknown posts for a moderator loses nothing and stops legitimate enquiries disappearing. If you run the list, this is the setting most worth changing.
Use delivery management if the list lives in Microsoft 365Distribution groups there are controlled through delivery management on the group object, not through list software. Adjust the setting for senders that must be authenticated, and expect the tenant to report refusals as 5.7.1 rather than as this code.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about SMTP error 5.7.2

Why can I email individual colleagues at that organisation but not their list?

Because the policy belongs to the list, not to the domain. Individual mailboxes accept mail from anybody; the list has its own membership check that runs first. Nothing about your domain reputation or your DNS records is involved.

Does the list owner get told that we tried?

Usually not. A rejection at this point happens before the message is queued for a moderator, so there is nothing for anybody to review. You have to tell the owner yourself, which is why these situations tend to persist for months.

Should we expect this code from a Microsoft 365 distribution group?

Rarely. Exchange Online reports a refused post to a restricted group as 5.7.1 in almost every case, so if you are troubleshooting a 365 group, work from that page instead. Seeing this specific code points at list software rather than at a tenant.

Related SMTP status codes

Source. SMTP error 5.7.2 Mailing list expansion prohibited is defined in RFC 3463 and registered in the IANA SMTP Enhanced Status Codes registry. The causes and fixes above are drawn from our own experience supporting UK business email systems.

All Email delivery & SMTP codes