Gmail: "This message is suspicious due to the very low reputation of the sending domain"

What Gmail says

Gmail uses two codes for this. 421 is temporary, 550 is permanent. Each comes in an IP form and a domain form.

421 4.7.0, IP

This message is suspicious due to the very low reputation of the sending IP address. To protect our users from spam, email sent from your IP address has been temporarily rate limited. For more information, go to Why has Gmail blocked my messages? (https://support.google.com/mail/answer/188131).

421 4.7.0, domain

This message is suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. For more information, go to Why has Gmail blocked my messages? (https://support.google.com/mail/answer/188131).

550 5.7.1, IP:

This message is likely suspicious due to the very low reputation of the sending IP address. To best protect our users from spam, the message has been blocked. For more information, go to Why has Gmail blocked my messages? (https://support.google.com/mail/answer/188131)

550 5.7.1, domain:

This message is likely suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. For more information, go to Why has Gmail blocked my messages? (https://support.google.com/mail/answer/188131)

Source: Google, Gmail SMTP errors and codes, sections 421 4.7.0 and 550 5.7.1, fetched 2026-10-08.

What it means, in plain words

This is not an authentication error. Google's text does not mention SPF, DKIM or DMARC. It names the low reputation of the sending domain or IP, and it does not say which signals produced it. Authentication is identity. This text is about reputation.

The domain form names the domain in your From address, so changing IPs or sending services does not change what it names.

Check your own side

• First confirm the identity column is clean, so you are not fixing two things at once: run the audit on the domain and read the SPF, DKIM and DMARC lines. If any fails, fix that first (the 5.7.26, 5.7.27, 5.7.30 and 5.7.40 pages).
• Then read Google Postmaster Tools for the domain. It is Google's own view of your domain's reputation and spam rate. The domain must be verified there; Google's Set up Postmaster Tools page shows the steps.
• Read your sending tool's bounce log for the same days: how many of these texts, from which sending path, to how many recipients.

The fix

There is no record to publish. The fix is a change in what you send and to whom, measured in Postmaster over days.

• Stop sending to any list that produced complaints or hard bounces. Remove every hard bounce. Send only to recipients who opted in and engaged recently.
• Lower the daily volume, hold it steady, and raise it slowly.
• Give every message a working one-click unsubscribe header and honour it the same day. Google's sender guidelines (https://support.google.com/mail/answer/81126) state a spam rate threshold; read the current number there.
• Separate your sending paths. Marketing or cold mail from a dedicated subdomain so the main domain's reputation is not spent on it.
• Watch Postmaster daily until the domain reputation reading improves. Then keep the volume discipline.

What this page covers: the authentication records the receiver checked and how to correct them. It does not predict where a message lands once accepted; authentication and inbox placement are separate questions.

FAQ

Is this an authentication error?

No. Google's text names reputation, not SPF, DKIM or DMARC. Check authentication first so you are not fixing two things at once.

My authentication is perfect. Why does Gmail say low reputation?

Authentication and reputation are separate. Authentication proves the mail is yours. Google's text for this error names reputation.

How long does it take?

Gmail's text gives no duration and neither does this page. Postmaster Tools is where the change shows; read it daily and judge from the trend, not from one day.