Error SMTP 550 when sending emails

Hahne Masley 0 Reputation points
2026-11-10T17:57:28.0033333+00:00

I'm having trouble sending emails through my Exchange account. Whenever I try to send an email to an external recipient, I sometimes receive a delivery failure notification with SMTP error 550. I'm not sure what is causing this issue.

Exchange Online
Exchange Online

A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.

0 comments

2 answers

Sort by: Most helpful
Most helpful Newest Oldest
  1. BabybooHN 3,895 Reputation points Independent Advisor
    2026-11-10T18:07:03.7+00:00

    Dear @Hahne Masley

    SMTP 550 means a mail server rejected the message. The number alone doesn’t tell us why. I’d start by opening the delivery failure notice and looking under Diagnostic information for administrators for the full line after Remote server returned. For example, 550 5.1.1 typically points to a recipient address problem, while 550 5.7.1 can indicate a permissions or mail policy restriction.

    Please refer to this document: Fix NDR error "550 5.7.1" in Exchange Online

    • If it happens with one recipient: Verify their address through another channel, then type it manually in a new message instead of selecting the saved Outlook suggestion.
    • If it happens with several recipients or domains: Ask your Exchange administrator to run Mail flow > Message trace in the Exchange admin center for one failed message. Give them the sender, recipient, and approximate send time so they can check where the message was rejected. Please refer to this document: Trace an email message in Exchange Online

    Could you share the full 550 error line with email addresses hidden, and let me know whether it affects one recipient, one domain, or several domains? That will help me suggest the right fix.

    Was this answer helpful?

    0 comments

  2. AI answer

    2026-11-10T17:57:45.27+00:00

    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:

    AI-generated content may be incorrect. Read our transparency notes for more information.

    Was this answer helpful?

Your answer

Answers can be marked as 'Accepted' by the question author and 'Recommended' by moderators, which helps users know the answer solved the author's problem.