What does “550 5.7.1 This message is likely suspicious due to the very low reputation of the sending domain” mean?

What does it mean?

Gmail judged the reputation of the sending domain to be very low, and blocked the message for that reason, not for a fault in this one message.

What exactly does the source say?

550 5.7.1 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?

Gmail's page writes domain-name and ip-address where your bounce shows your own domain and the sending address.

421 4.7.0 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?

Why would a small firm see it?

Spam sent from a stolen mailbox at your firm, a newsletter sent to an old list, or mail forged in your name by someone else when your domain has no DMARC policy at enforcement.

How do I fix it?

1. Start herep=nonereports only
2. p=quarantinesuspicious mail to spam
3. Where to aimp=rejectforged mail refused

v=DMARC1; p=none; rua=mailto:[email protected]

v=DMARC1; p=quarantine; rua=mailto:[email protected]

v=DMARC1; p=reject; rua=mailto:[email protected]

1. Check every mailbox's Sent folder for mail nobody at the firm wrote.Change those passwords and turn on two-step sign-in.
2. Make sure every service that sends as your domain passes SPF and DKIM.
3. Publish DMARC and raise it to p=reject once every service passes, so mail forged in your name is refused.
4. Send newsletters only to people who asked, with an unsubscribe link.

Use your own domain and a mailbox someone reads where the record shows yourfirm.example.

What else do the sources say?

Keep spam rates reported in Postmaster Tools below 0.3%.