SMTP 550 5.7.12
Sender not authenticated by organization (Microsoft 365)
550 5.7.12 means the recipient's Microsoft 365 address only accepts mail from inside its own organisation, so mail from outside is rejected. Related codes 5.7.133, 5.7.134 and 5.7.136 say the same for groups, mailboxes and mail users. Only the recipient's admin can change it.
Should you remove the address?
Remove the address from your list
Remove it from external sends. It will reject anything from outside the organisation for as long as the setting stays.
Block. Retrying changes nothing until the cause on your side is fixed.
What causes it
- The recipient mailbox or group only accepts mail from inside its organisation.
- It is a shared or departmental address not meant for outside mail.
- An admin restricted external senders after spam or phishing.
How to fix it
In order — try the first one first.
- Contact the person through another channel or a different address.
- If you have a relationship with the organisation, ask their admin to allow external senders for that address.
Who sends it
Microsoft 365 and Exchange Online.
Microsoft 365
Provider's description Sender was not authenticated by organization
Microsoft: the recipient address is set up to reject messages sent from outside its organisation.
Microsoft 365
Provider's description Sender not authenticated for group
The 5.7.133 version, for groups.
Common questions
- What does SMTP 550 5.7.12 mean?
- 550 5.7.12 means the recipient's Microsoft 365 address only accepts mail from inside its own organisation, so mail from outside is rejected. Related codes 5.7.133, 5.7.134 and 5.7.136 say the same for groups, mailboxes and mail users. Only the recipient's admin can change it.
- Should I retry a 550 5.7.12 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 5.7.12?
- Contact the person through another channel or a different address.
- Is 550 5.7.12 a hard bounce or a soft bounce?
- 550 5.7.12 is a block rather than a classic hard or soft bounce. The rejection is about your sending (authentication, reputation, content or the recipient's policy), not about whether the address exists. Retrying changes nothing until the cause on your side is fixed.
- Should I remove the address after a 550 5.7.12 bounce?
- Remove it from external sends. It will reject anything from outside the organisation for as long as the setting stays.
- Will verifying my list prevent 550 5.7.12?
- Not reliably. The address exists, so a mailbox check may accept it; the restriction only shows at delivery.
Clean the list before the next send.
A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.
- A real SMTP mailbox check on every row, before anything is sent
- Dead and mistyped addresses split out, so they never reach your sending tool
- Catch-all, disposable and role addresses flagged alongside
One plan, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.
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 5.1.8 — bad outbound sender (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
- 550 5.5.0 — Mailbox unavailable (Outlook.com, Hotmail)
- 550 5.7.513 — Blocked by the recipient's own block list (Microsoft 365)
Sources
- Source: Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online
- Source: RFC 3463: Enhanced Mail System Status Codes
Provider wording is quoted only where the provider publishes it.
Last reviewed 2026-10-22