550 5.1.0: sender denied
The destination server refused messages from you or for this recipient, often because the sender is blocked or a cached address is stale.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.1.0
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
550 5.1.0 Sender denied
What it means
Microsoft describes 5.1.0 as a refusal by the destination server to accept messages from the sender or for the recipient. Your address, your organization, or Exchange Online as a whole may be blocked by the recipient. Another documented cause is a stale Auto-Complete entry: when a recipient moves between on-premises Exchange and Exchange Online, the address stays the same but internal identifiers change. It also happens when someone saves a message to a file, opens it offline and replies. A related diagnostic, Recipient address rejected: SPF Permanent Error, points at the sender's SPF record.
Common causes
- You are on the recipient's personal block list.
- Your Outlook Auto-Complete entry for the recipient is outdated after a mailbox move.
- You replied to a message saved to a file and opened offline, which keeps only an internal address.
- Your domain's SPF record does not cover every source of your mail.
How to fix it
- 01If others at the same domain receive your mail, ask this recipient by another channel whether they have blocked you.
- 02Delete the recipient from your Auto-Complete list, type the full address again and resend.
- 03Reply from the mailbox rather than from a saved copy of the message.
- 04Admins: check that the SPF record for your domain includes all your sending sources.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Fix NDR error "550 5.1.0" in Exchange Onlinelearn.microsoft.com
- Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
Related codes
- 550 5.1.1Recipient address not found.The address does not exist in the destination system, is mistyped, or is cached in Outlook with details that are no longer valid.
- 5.1.0Address problem, unspecified.Something about an address in the message, the sender's or a recipient's, made the server refuse it, without saying exactly what was wrong.
Further reading
All 111 Microsoft codesEvery code in the wikiPaste your own bounce