SMTP 550 5.7.513
Blocked by the recipient's own block list (Microsoft 365)
550 5.7.513 means the recipient's organisation added your sending IP to its own block list in Microsoft 365. It is not Microsoft's block but theirs. Only that organisation can remove it, so contact the recipient another way if you think it was a mistake, and stop sending there until then.
Should you remove the address?
Suppress the address if it keeps failing
Suppress the whole domain for now. Every address there will reject you until their admin lifts the block.
Block. Retrying changes nothing until the cause on your side is fixed.
How it appears
Service unavailable, Client host <IP> blocked by <recipient domain> using Customer Block list (AS16012607)
What causes it
- The recipient's admin added your sending IP to their own block list.
- Another sender on a shared IP caused the block.
- Earlier mail from you was reported or judged unwanted by that organisation.
How to fix it
In order — try the first one first.
- Stop sending to that domain.
- If the mail was wanted, ask the recipient by phone or another channel to have their admin remove the block.
- If you share the IP with others, the block may have been triggered by another sender. A dedicated IP or a different sending service avoids that.
Who sends it
Microsoft 365 tenants.
Microsoft 365
Reply text Service unavailable, Client host <IP> blocked by <recipient domain> using Customer Block list (AS16012607)
Microsoft's advice: contact the recipient directly and ask them to remove it.
Common questions
- What does SMTP 550 5.7.513 mean?
- 550 5.7.513 means the recipient's organisation added your sending IP to its own block list in Microsoft 365. It is not Microsoft's block but theirs. Only that organisation can remove it, so contact the recipient another way if you think it was a mistake, and stop sending there until then.
- Should I retry a 550 5.7.513 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 5.7.513?
- Stop sending to that domain.
- Is 550 5.7.513 a hard bounce or a soft bounce?
- 550 5.7.513 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 5.7.513 bounce?
- Suppress the whole domain for now. Every address there will reject you until their admin lifts the block.
- Will verifying my list prevent 550 5.7.513?
- Not directly. This code is about your authentication, IP or domain reputation, so verification will not lift it. A clean list still matters, because mailbox providers weigh how often a sender hits addresses that don't exist.
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 5.7.12 — Sender not authenticated by organization (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