Permanent failure — do not retry
SMTP 550 5.7.515
authentication required for high-volume senders (Outlook.com)
SMTP 550 5.7.515 is Outlook.com rejecting mail from a high-volume sender that does not meet its authentication requirements. Microsoft requires domains sending more than 5,000 messages a day to Outlook.com, Hotmail and Live addresses to pass SPF, DKIM and DMARC. Fix the records and resend.
How it appears
550 5.7.515 Access denied, sending domain [example.com] doesn't meet the required authentication level.
What causes it
- SPF, DKIM or DMARC is missing or failing for the sending domain.
- DMARC is not published, or does not align with SPF or DKIM.
- A new sending service was added without authentication.
How to fix it
In order — try the first one first.
- Publish SPF for every service that sends as the domain.
- Set up DKIM signing on every sending service.
- Publish a DMARC record, at least p=none, aligned with SPF or DKIM.
- Run a domain audit to confirm all three pass.
Common questions
- What does SMTP 550 5.7.515 mean?
- SMTP 550 5.7.515 is Outlook.com rejecting mail from a high-volume sender that does not meet its authentication requirements. Microsoft requires domains sending more than 5,000 messages a day to Outlook.com, Hotmail and Live addresses to pass SPF, DKIM and DMARC. Fix the records and resend.
- Should I retry a 550 5.7.515 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 5.7.515?
- Publish SPF for every service that sends as the domain.
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 5.7.1 S3140 — IP range on Outlook.com block list
Last reviewed 2026-09-11