SMTP 550 5.5.0
Mailbox unavailable (Outlook.com, Hotmail)
550 5.5.0 "Requested action not taken: mailbox unavailable" is what Outlook.com returns when a Hotmail, Outlook or Live address does not exist. Microsoft describes it as similar to 5.1.10. Treat it as a hard bounce and remove the address unless you spot a typo.
Should you remove the address?
Remove the address from your list
Yes, when it comes from Outlook.com with the "mailbox unavailable" text. The mailbox does not exist, so keep it off future sends.
Hard bounce. Do not retry. It will fail every time.
How it appears
550 5.5.0 Requested action not taken: mailbox unavailable
What causes it
- The Outlook.com, Hotmail or Live address was mistyped.
- The account was closed, often after a long period of inactivity.
- The address never existed and was entered to get past a form.
How to fix it
In order — try the first one first.
- Check the local part for typos. Consumer Outlook addresses are often typed from memory.
- If it is correct, remove it. Outlook.com retires inactive accounts, and the address then returns this reply.
- If you see 550 5.5.0 from a non-Microsoft server, read the text: there it can mean a malformed command rather than a missing mailbox.
Who sends it
Outlook.com, which hosts @outlook.com, @hotmail.com, @live.com and @msn.com addresses. Other servers occasionally use 550 5.5.0 for protocol errors, which is why reading the reply text matters.
Outlook.com
Reply text 550 5.5.0 Requested action not taken: mailbox unavailable
Microsoft says this appears when the recipient's domain is @hotmail.com or @outlook.com and the address wasn't found by SMTP address lookup.
Common questions
- What does SMTP 550 5.5.0 mean?
- 550 5.5.0 "Requested action not taken: mailbox unavailable" is what Outlook.com returns when a Hotmail, Outlook or Live address does not exist. Microsoft describes it as similar to 5.1.10. Treat it as a hard bounce and remove the address unless you spot a typo.
- Should I retry a 550 5.5.0 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 5.5.0?
- Check the local part for typos. Consumer Outlook addresses are often typed from memory.
- Is 550 5.5.0 a hard bounce or a soft bounce?
- 550 5.5.0 is a hard bounce. The receiving side has said the address can't receive mail, so a retry won't deliver it. Do not retry. It will fail every time.
- Should I remove the address after a 550 5.5.0 bounce?
- Yes, when it comes from Outlook.com with the "mailbox unavailable" text. The mailbox does not exist, so keep it off future sends.
- Will verifying my list prevent 550 5.5.0?
- Yes. A mailbox check asks the recipient's server whether the address exists without sending anything, so addresses that would return this code are flagged before the send instead of bouncing against your domain.
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 5.7.12 — Sender not authenticated by organization (Microsoft 365)
- 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