Email is being recognized as spam and not allowing to send emails

A Microsoft 365 administrator must secure the affected mailbox and then unblock it under Microsoft Defender → Email & collaboration → Review → Restricted entities.

This message normally corresponds to 550 5.1.8 Access denied, bad outbound sender, which Exchange Online returns when an account has exceeded outbound sending limits and is restricted from sending. It is not a Gmail safe-sender or Outlook Web browser issue.^1^

If you are the Microsoft 365 administrator

1. Secure the account before unblocking it
   • Review Sent Items and Deleted Items for messages the user did not send.
   • Check for unknown Inbox rules or forwarding addresses.
   • Reset the password, revoke active sessions, and remove unfamiliar authentication methods or application access.
   • Enable multifactor authentication if it is not already enabled.
   A blocked mailbox, suspicious messages, unknown forwarding, and suspicious Inbox rules can indicate account compromise.^2^
2. Review the restriction
   • Open the Microsoft Defender portal.
   • Go to Email & collaboration → Review → Restricted entities.
   • Find the affected mailbox. Its Entity value should be Mailbox.
   • Select it and choose Unblock.
3. Complete the Unblock user wizard
   • Review the recommendations.
   • Confirm the account has been secured.
   • Select Next → Submit → Yes.
   Removing a restriction requires Organization Management or Security Administrator permissions. Restrictions are usually cleared within one hour, although transient issues can take up to 24 hours.^3^

Administrators can alternatively verify and remove the restriction through Exchange Online PowerShell:

# List restricted senders
Get-BlockedSenderAddress

# Inspect the affected mailbox
Get-BlockedSenderAddress -SenderAddress [Email] |
    Format-List

# Run only after the account has been secured
Remove-BlockedSenderAddress -SenderAddress [Email]

If you are not the administrator

1. Change the mailbox password immediately if you notice unfamiliar sent messages, rules, forwarding, or sign-ins.
2. Send the complete delivery-failure report to your organization’s IT or Microsoft 365 administrator.
3. Ask them to check your mailbox on the Restricted entities page and unblock it after completing the security review.

Do not repeatedly resend the message or ask the Gmail recipient to add you as a safe sender; this rejection occurs on the Microsoft 365 sending side before normal recipient spam handling.

If the mailbox is not listed under Restricted entities, the administrator should inspect the complete Diagnostic information for administrators section of the bounce message. A different SMTP code requires a different remediation; the procedure above specifically applies to the unrecognized-sender/550 5.1.8 failure.

References

1. Fix NDR error "550 5.1.8" in Exchange Online
2. Respond to a compromised cloud email account
3. Remove blocked users from the Restricted entities page