Few bounces cause as much panic as this one:

550-5.7.1 Our system has detected that this message is likely suspicious due to the very low reputation of the sending domain.

It means Gmail is no longer filtering your mail into spam. It is refusing it. Here is what that means, the mistake that turns a bad week into a dead domain, and how to come back.

What the error actually says

Three parts matter:

  • 550 is a permanent failure. Retrying the same message will not help.
  • 5.7.1 is a policy rejection: the receiver chose to refuse.
  • "Reputation of the sending domain" means the verdict is about your domain, built from the history of everything sent from it. Not this one email, not this one recipient.

A related temporary version, a 421 about the reputation of the sending IP, means Gmail is throttling rather than refusing. Treat it as an early warning.

How domains get here

Almost always a combination of:

  • Sending cold at volume from a new or quiet domain with no history.
  • High bounce rates from unverified lists.
  • Spam complaints above Gmail’s threshold (Google asks bulk senders to stay under 0.3%, and well under 0.1% is the real target).
  • Missing or broken authentication: no DKIM, no DMARC, an SPF record past its 10-lookup limit.
  • Low engagement: lots of mail nobody opens or replies to.

The mistake that makes it worse

Your sending server often reports these messages as sent. It accepted them; Gmail refused them later, during delivery, and a bounce notice ("Delivery Status Notification", "failure notice") came back to your inbox.

If nothing reads those notices, the campaign carries on. Every fifteen minutes another batch goes out and gets refused, and each refusal tells Gmail it was right. We watched exactly this on a customer’s warm-up: their own server accepted everything, Gmail refused everything, and the inbox filled with hundreds of failure notices while the schedule kept sending.

This is why Leads Ranger’s warm-up reads the bounce notices that come back, works out the cause, and rests the sender from that provider only the moment a reputation refusal appears, restarting its ramp at a few messages a day instead of sending into the block.

The recovery plan

1. Stop sending to Gmail from that domain. Today. Pause cold campaigns entirely if most of your list is on Gmail or Google Workspace.

2. Fix authentication. SPF valid and under 10 lookups, DKIM signing on, DMARC published. Our free deliverability checker shows all three in one go.

3. Clean the list. Remove every address that bounced, verify everything else, and drop contacts who have never engaged.

4. Rest. A week or two of near-silence toward Gmail lets the worst signals age.

5. Warm back up slowly. Real conversations with engaged mailboxes, a few a day, growing gradually. Watch Google Postmaster Tools if you have the volume to register.

6. Resume at a fraction of your old volume, to your most engaged and best-verified contacts first, and grow only while bounces and complaints stay low.

Prevent it next time

  • Send cold email from a separate sending domain, never your main one.
  • Warm every new mailbox before outreach. See the warm-up guide.
  • Verify every list. See email verification.
  • Keep volumes per mailbox modest and spread sending across several mailboxes.
  • Read your bounces, or use a tool that does.

A 550 5.7.1 feels final, but it is a reputation, and reputations recover. The domains that do not come back are the ones that kept sending into the refusal.