550 5.1.8 Access denied, bad outbound sender
The sending account was blocked for spam
Seen in Microsoft 365, Exchange Online (outbound)
Microsoft 365 put the sender on its Restricted users list because the account sent mail that looked like spam, so it can no longer send.
Common causes
- The account was compromised and used to send spam or phishing.
- A legitimate bulk send from a normal mailbox exceeded outbound spam thresholds.
How to fix it
- Secure the account: reset the password, revoke sessions, enforce MFA and remove unknown inbox rules.
- An admin can then release the user from Restricted entities in the Microsoft Defender portal.
- Send bulk or marketing mail through a dedicated bulk email service, not a user mailbox.
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Related errors
- 550 5.4.1 Recipient address rejected: Access denied · Microsoft 365 rejected an unknown recipient at the edge
- 550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP · Microsoft 365 blocked the tenant's outbound mail
- 530 5.7.57 Client not authenticated to send anonymous mail during MAIL FROM · The app sent to smtp.office365.com without logging in
- 535 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Tenant · Microsoft 365 refused the SMTP AUTH login
- 550 5.7.520 Access denied, Your organization does not allow external forwarding · Automatic forwarding to an outside address is blocked
- 451 4.7.500 Access denied, please try again later · Microsoft 365 is temporarily throttling the sending IP
- 552 5.2.2 The recipient's inbox is out of storage space · The recipient's mailbox is full
- 552 5.3.4 Your message exceeded Google's message size limits · The message is larger than the server accepts
- 535 5.7.8 Username and Password not accepted · The SMTP login was rejected
- 450 4.2.0 <user@example.com>: Recipient address rejected: Greylisted · The message was greylisted; it will be accepted on retry
- 553 5.7.1 <user@example.com>: Sender address rejected: not owned by user · You logged in as one user but sent as another address
- 530 5.7.0 Must issue a STARTTLS command first · The server requires TLS before it accepts mail