550 5.1.0: sender denied
The destination server won't accept messages from you, or messages for that recipient. Common causes are a block list, a stale cached address, or an incomplete SPF record.
Recipient address rejected: SPF Permanent Error.
One diagnostic line Microsoft says this NDR may contain; the user section may read "Address Rejected".
What 550 5.1.0 means
Microsoft describes 5.1.0 as the destination email server refusing messages from you (the sender) or messages for the recipient. That can happen when messages from you are being blocked by the recipient, whether it's your address, your Exchange Online organization, or even all of Exchange Online. Its NDR reference adds that the recipient address may be incorrectly formatted or couldn't be correctly resolved, and that a common trigger is replying to a message that was saved to a file and opened offline in Outlook.
Start with the simple checks Microsoft lists. If you can send to other people at the same domain, you may be on the recipient's personal block list; ask them by phone or in person. You may also have a bad entry in your Auto-Complete list (the nickname cache), for example after the recipient's mailbox moved between on-premises Exchange and Exchange Online. Remove the cached entry, type the address fresh, and resend.
For admins, Microsoft points to SPF: your domain's SPF record might be incomplete and not include every source of mail for your domain. The diagnostic text "Recipient address rejected: SPF Permanent Error" is one of the details Microsoft says can appear with this code. An SPF permerror is often a syntax problem, more than one SPF record, or too many DNS lookups.
Likely causes
- Your address or organization is on the recipient's block list.
- A stale Auto-Complete (nickname cache) entry for the recipient, often after a mailbox move.
- Replying to a message saved to a file and opened offline, so the recipient can't be resolved.
- An incomplete or broken SPF record for your domain (SPF permanent error).
How to fix it
If you sent the message
- Check the recipient's address, then remove their Auto-Complete entry and type the address again.
- If others at the same domain receive your mail, ask the recipient whether they blocked you.
- Admins: check that your SPF record includes every service that sends as your domain and has no syntax errors or extra records.
If you're the recipient or their admin
- Check personal and organization block lists for the sender's address or domain.
- If you publish an SPF record, make sure it's valid; receivers reject on SPF permanent errors.
Frequently asked questions
Does 550 5.1.0 mean the recipient blocked me?
It can. Microsoft lists the recipient's personal block list as a likely cause when you can still send to other people at the same domain. Other causes are a stale cached address in Outlook or a broken SPF record for your domain.
How does SPF cause a 5.1.0 sender denied bounce?
If your SPF record is incomplete or invalid, the receiving server can reject your mail. Microsoft says the NDR may then show "Recipient address rejected: SPF Permanent Error". Fix the record so it lists every sender and has no syntax errors.
Sources
- Fix NDR error 550 5.1.0 in Exchange Online (Microsoft Learn)
- Email nondelivery reports (NDRs) and SMTP errors in Exchange Online (Microsoft Learn)
- RFC 7208, section 2.6.7: SPF permerror
Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.
Related codes
- 550 5.1.1 550 5.1.1: bad destination mailbox address
- 550 5.1.10 550 5.1.10: recipient not found (Microsoft 365)
- 550 5.7.23 550 5.7.23: SPF validation failed
- 550 5.1.20 550 5.1.20: multiple From addresses are not allowed without Sender address
- 550 5.1.8 550 5.1.8: access denied, bad outbound sender
- 550 5.6.11 550 5.6.11: invalid characters (bare line feeds)
- 550 5.0.350 550 5.0.350: generic error from the recipient's server
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