Error SMTP 550 when sending emails

SMTP 550 is a general mail rejection code. In Exchange Online, the exact cause depends on the full enhanced status code that appears in the non-delivery report (NDR).

Common 550-related causes include:

1. 550 5.7.1 Delivery not authorized
   The sender is not allowed to send to that recipient. This often happens when the recipient is a distribution group or other recipient configured to accept mail only from specific senders.
2. 550 5.7.1 Unable to relay
   The sending system is not permitted to relay through the receiving system. This can happen when:
   • the sending system does not have permission to relay,
   • the receiving system does not accept mail for the recipient domain,
   • or an MX record points to a server that is not configured to accept that domain.
3. 550 5.7.1 Client was not authenticated
   The receiving server requires authentication before accepting the message, but the sending system did not authenticate.
4. 550 / 553 relay-prohibited or relaying denied
   This means the outgoing SMTP server did not recognize the sender as an authorized user. Microsoft Support notes this commonly occurs when sending through an SMTP server that does not allow relaying for that account or connection.
5. 550 5.1.8 Access denied, bad outbound sender
   The sending account was blocked for sending too much spam, often because the account was compromised.

How to narrow it down:

1. Open the delivery failure message and find the full code, such as 5.7.1, 5.1.8, or 5.2.0.
2. Check whether the failure happens:
   • only to one external recipient,
   • to all external recipients,
   • or only from one mailbox.
3. Match the code to the likely cause:
   • 5.7.1 = authorization, relay, or authentication problem.
   • 5.1.8 = outbound sender blocked due to suspected spam.
   • relay prohibited / relaying denied / 553 = SMTP relay configuration or authorization issue.

If the issue is 550 5.7.1 and only one recipient is affected, the recipient may be restricted from accepting external mail or may require authorized senders.

If the issue is 550 5.1.8, the mailbox may be restricted from sending outbound mail and the account should be checked for compromise before being unblocked.

If the NDR includes wording like authentication required for a public folder, an Exchange admin can allow external senders by going to Exchange admin center > Public folders > Public folders > select the folder > Edit > Mail flow settings > Message Delivery Restrictions, then clearing Require that all senders are authenticated and selecting All senders.

The next step is to use the exact NDR text to identify which 550 variant is occurring, because SMTP 550 by itself is not enough to determine the root cause.

References:

• I receive a 550, 553, or relay-prohibited error when sending email messages | Microsoft Support
• Email non-delivery reports and SMTP errors in Exchange Online
• Fix NDR error "550 5.7.1" in Exchange Online