Emails are being blocked as spam

Desert Life Club 0 Reputation points
2026-10-10T19:09:34.8766667+00:00

We are a small club with 195 members, and our emails only go to our members that authorized to be sent emails. All of a sudden all of our emails being sent to any member are being blocked as spam and not delivered

Outlook | Web | Outlook.com | Email
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  1. AI answer

    2026-10-10T19:09:48.7233333+00:00

    For a small club sending to 195 opted-in members, the most likely causes are:

    1. The account was restricted for suspected spam or exceeded outbound sending limits
      • If sending is blocked for all recipients, check whether the sender appears on the Restricted entities page in the Microsoft Defender portal: https://security.microsoft.com/restrictedusers.
      • A blocked sender commonly gets an NDR with 5.1.8 (bad outbound sender).
      • Before unblocking, review whether the account was compromised:
        • Check sign-in activity for suspicious access.
        • Check for unknown inbox rules or forwarding rules.
        • Review recent sent items.
      • If the account was compromised, change the password and enable MFA before removing the restriction.
    2. The messages are being classified as spam because of authentication or sender reputation
      • Verify that SPF, DKIM, and DMARC are correctly configured for the sending domain.
      • Verify the sending IP has a valid reverse DNS (PTR) record.
      • If mail is sent through a third-party filtering service before Microsoft 365, configure Enhanced Filtering for Connectors so the original source IP is preserved.
    3. The message content is triggering spam filtering
      • Test with a very simple plain-text message.
      • Remove anything that commonly triggers filtering, such as:
        • excessive links
        • URL shorteners
        • embedded scripts
        • image-only content
        • attachments or repeated promotional text
      • If simple plain-text messages send successfully, add content back gradually to identify the trigger.
    4. A policy or block list is overriding delivery
      • In the Exchange admin center, review Mail flow rules.
      • Check the applicable anti-spam policy for blocked senders or domains.
      • If messages are reaching recipients but landing in Junk, ask recipients to check their Blocked Senders list and submit false positives to Microsoft.
    5. If the issue is only with messages sent to Microsoft 365 recipients from an external sender IP
      • Check the NDR for 5.7.606-649 (banned sending IP).
      • If that code appears, use the delist portal at https://sender.office.com/.

    A practical order to troubleshoot is:

    1. Send a plain-text test message to one member and review the exact NDR code.
    2. If the NDR is 5.1.8, check Restricted entities and investigate compromise.
    3. If mail is delivered to Junk instead of blocked, inspect message headers and run Message trace in Exchange admin center > Mail flow > Message trace.
    4. Verify SPF/DKIM/DMARC for the club domain.
    5. Review mail flow rules and anti-spam policy overrides.

    If the sender is using Outlook.com as an end-user account and outbound spam protection is blocking sending, only an admin can review and resolve those issues in a managed Microsoft 365 organization.

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

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