Permanent failure — do not retry
SMTP 550 5.1.8
bad outbound sender (Microsoft 365)
SMTP 550 5.1.8 means Microsoft 365 has blocked your own account from sending. The error reads "Access denied, bad outbound sender": the account went over sending limits or sent mail that looked like spam, which Microsoft says usually means it was compromised. It is not about the recipient.
How it appears
550 5.1.8 Access denied, bad outbound sender
Remote Server returned '550 5.1.8 Access denied, bad outbound sender'
What causes it
- The account was compromised and used to send spam.
- The account sent more mail than Exchange Online's sending limits allow, for example a bulk send from a normal mailbox.
- A script or app using the account's credentials sent a burst of mail.
How to fix it
In order — try the first one first.
- Secure the account first: reset the password, review sign-ins and remove any forwarding rules or apps you did not set up.
- An admin then releases the user from the Restricted entities page in Microsoft Defender. Microsoft says this usually takes effect within an hour.
- Send bulk or cold mail through a dedicated sending service, not a Microsoft 365 mailbox.
- Do not remove any recipients because of 5.1.8. Nothing is wrong with them.
Common questions
- What does SMTP 550 5.1.8 mean?
- SMTP 550 5.1.8 means Microsoft 365 has blocked your own account from sending. The error reads "Access denied, bad outbound sender": the account went over sending limits or sent mail that looked like spam, which Microsoft says usually means it was compromised. It is not about the recipient.
- Should I retry a 550 5.1.8 error?
- No. 550 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 550 5.1.8?
- Secure the account first: reset the password, review sign-ins and remove any forwarding rules or apps you did not set up.
Other 550 replies
- 550 5.1.1 — mailbox does not exist
- 550 5.7.1 — message refused by policy
- 550 5.4.1 — recipient address rejected, access denied
- 550 5.2.1 — mailbox disabled, not accepting messages
- 550 — mailbox unavailable
- 550 5.1.0 — address rejected, other address status
- 550 5.1.10 — recipient not found (Microsoft 365)
- 550 5.4.5 — daily sending limit exceeded (Gmail)
- 550 4.4.7 — message expired in the queue
- 550 5.7.25 — no PTR record for sending IP (Gmail)
- 550 5.7.26 — blocked, sender unauthenticated (Gmail)
- 550 5.7.27 — SPF authentication failed (Gmail)
- 550 5.7.28 — unusual rate of unsolicited mail (Gmail)
- 550 5.7.23 — SPF violation (Microsoft 365)
- 550 5.7.124 — sender not in allowed senders list (Microsoft 365)
- 550 5.7.509 — DMARC failed, policy reject (Microsoft 365)
- 550 5.7.606-649 — banned sending IP (Microsoft)
- 550 5.7.705 — tenant has exceeded threshold (Microsoft 365)
- 550 5.7.708 — traffic not accepted from this IP (Microsoft 365)
- 550 5.7.750 — blocked from sending from unregistered domains (Microsoft 365)
- 550 5.7.515 — authentication required for high-volume senders (Outlook.com)
- 550 5.7.1 S3140 — IP range on Outlook.com block list
Last reviewed 2026-09-11