Short answer: The receiving server refused the message because of a security or policy rule. The most common reasons are a blocklisted sending IP, a server that won't relay for you, or a recipient that only accepts mail from certain senders.
550 5.7.1 bounce message examples
The same code comes with different wording on different servers. In the bounce (non-delivery report) you may see one of these lines:
550 5.7.1 Service unavailable; Client host [203.0.113.10] blocked using Spamhaus (Microsoft) 550 5.7.1 Unable to relay 550 5.7.1 Relaying denied 550 5.7.1 [IP] Our system has detected that this message is likely unsolicited mail. (Gmail) 550 5.7.1 Delivery not authorized, message refused 550 5.7.1 RESOLVER.RST.AuthRequired; authentication required (Microsoft 365)
Type: Permanent error. The sending server won't retry; fix the problem and send the message again.
What causes it?
| Cause | Explanation |
|---|---|
| IP on a blocklist | If the text mentions "blocked using", "Spamhaus" or "RBL", the sending IP is listed in a DNSBL. |
| Relay denied | "Unable to relay" / "Relaying denied": your email client is sending without SMTP authentication, or using the wrong server. |
| Recipient accepts internal senders only | Microsoft 365 distribution groups reject external mail by default: RESOLVER.RST.AuthRequired. |
| Content or reputation filter | Gmail's "likely unsolicited mail" means the content or the sender's reputation was judged to be spam. |
| Sender blocked | The recipient may have added you to their own block list. |
What to do as the sender
- Read the whole message; the list name after "blocked using" tells you where the problem is.
- Check your mail server's IP with the blacklist check. If it is listed, find the source and follow the delisting steps.
- For "relay" messages, make sure your email client has authentication enabled for the outgoing (SMTP) server and uses port 587 or 465.
- For Gmail rejections, review your SPF, DKIM and DMARC records and your content (URL shorteners, image-only messages, missing unsubscribe link).
What the receiving side's admin should check
- If a distribution group should receive external mail, turn on "Allow external senders to email this group" in Microsoft 365.
- If your own server returns relay denials, make sure users send with SMTP AUTH; never open relaying to everyone by IP.
- If you are blocking a legitimate sender by mistake, review your block list and the RBLs you use.
Good to know
Under RFC 3463, 5.7.1 means "delivery not authorized, message refused", a deliberately generic code, which is why it is used for so many different problems. The text after the code matters more than the code itself. Microsoft uses 5.7.23 for SPF failures, and Gmail uses 5.7.26 for authentication rejections.
Protect your inbox from spam, malware and phishing: point your MX record to Spambil and try it free for 7 days. No credit card required.
Frequently asked questions
Does 550 5.7.1 mean my mail server is blacklisted?
Not always. If a list name (Spamhaus, SpamCop, Barracuda) appears, yes. If it says "relay", the problem is authentication; if it says "AuthRequired", the recipient group doesn't accept external senders.
I can't email one particular company but everyone else works. Why?
That company's blocklist, content filter or block list may be rejecting you. Share the explanation text from the bounce with their IT team.