Exchange Online Email delivery

550 5.7.1: the message was rejected (not allowed to send or relay)

Likely causes

• The recipient is a group that only accepts mail from members or approved senders.
• The recipient's mailbox or public folder rejects mail from external or unauthenticated senders.
• A mail flow (transport) rule rejected the message.
• The sending IP address is on Microsoft's blocklist.
• An MX record points to a server that doesn't accept mail for that domain.

How to fix it

1. Read the whole bounce. The text after 5.7.1 (for example Unable to relay or Delivery not authorized) tells you which case you have.
2. If you sent to an internal group, ask the group owner to allow you to send, or to approve your message if the group is moderated.
3. If the recipient is external, contact them another way and ask them to pass the bounce to their email admin. You usually can’t fix it from your side.
4. If the bounce says blocked using Blocklist 1, the sender’s admin can request removal through the Microsoft delist portal or by forwarding the NDR to delist@microsoft.com.
5. Admins: confirm the domain shows as Healthy under Settings > Domains, that it has exactly one MX record, and that its SPF record lists every service that sends as your domain.

Checked against: learn.microsoft.com, learn.microsoft.com