Bounce codes · Permanent

550 5.7.1: Delivery not authorized, message refused

5.7.1 is the generic "delivery not authorized" rejection. From Gmail it usually means the message looked like spam or came from a very low-reputation IP or domain. From Microsoft 365 it usually means a recipient restriction, a transport rule, or an unauthenticated relay attempt. The text after the code tells you which.

The exact messages

Gmail / Google Workspace

550 5.7.1 This message is likely unsolicited email.

Gmail / Google Workspace

550 5.7.1 This message is likely suspicious due to the very low reputation of the sending IP address.

Gmail / Google Workspace

550 5.7.1 This message is likely suspicious due to the very low reputation of the sending domain.

Gmail / Google Workspace

550 5.7.1 The user or domain that you are sending to (or from) has a policy that prohibits the email.

Gmail / Google Workspace

550 5.7.1 Messages missing a valid Message-ID: header are not accepted.

Gmail / Google Workspace

550 5.7.1 Messages missing a valid address in the From: header, or having no From: header, are not accepted.

Gmail / Google Workspace

550 5.7.1 Messages with multiple addresses in the From: header are not accepted.

Microsoft 365 / Exchange Online

5.7.1 Delivery not authorized

Microsoft 365 / Exchange Online

5.7.1 Unable to relay

Microsoft 365 / Exchange Online

5.7.1 Client was not authenticated

Any mail server (RFC standard meaning)

5.7.1 Delivery not authorized, message refused

Why it happens

Gmail: spam-like content or a sending domain or IP whose reputation has collapsed.
Microsoft: the recipient (often a distribution group) accepts mail only from approved senders, or a transport rule blocked it.
Microsoft: a server tried to relay through a system that does not accept mail for that domain, or skipped authentication.

How to fix it

Read the full text after the code to see which variant you hit.
For reputation variants: stop sending from that domain, check blocklists and Postmaster Tools, and rebuild with low volume.
For relay or authentication variants: authenticate your SMTP connection, or fix the MX record that points at the wrong server.

If you send cold email

A Gmail "likely unsolicited email" rejection on outreach is a content-and-reputation verdict. Plain-text messages, no link tracking, verified lists and lower volume are the levers that move it.

How WarmHawk handles it

WarmHawk pauses a mailbox on its own once its hard-bounce rate passes 5% over at least 20 sends, and pauses a campaign at that campaign’s own bounce threshold, so a bad list stops before it drags the domain down. Every sending domain is re-checked against DNS blocklists hourly, and a mailbox only reaches campaign volume after at least 14 days of warmup with a 90% inbox rate. A mailbox whose inbox rate falls below 70% goes back to warmup. How WarmHawk works →

Check your domain now

These free checkers read your live DNS: no account, up to 15 domains at once.

Email blacklist checker →Full domain health check →

Related bounce codes

Sources, checked 2026-09-29: Google Workspace: Gmail SMTP errors and codes · Microsoft Learn: NDRs and SMTP errors in Exchange Online · RFC 3463: Enhanced Mail System Status Codes. Have a different bounce? Paste it into the decoder →

Questions

5.7.1 questions

What does 5.7.1 mean?+

Is 5.7.1 a temporary or permanent error?+

Permanent. The message will not be retried, and sending it again unchanged will fail the same way until the cause is fixed.

How do I fix 5.7.1?+

Read the full text after the code to see which variant you hit. For reputation variants: stop sending from that domain, check blocklists and Postmaster Tools, and rebuild with low volume. For relay or authentication variants: authenticate your SMTP connection, or fix the MX record that points at the wrong server.