Gmail "421-4.7.0 ... very low reputation of the sending IP address": how to fix it

What it means

Gmail is not refusing your email for good. It is asking your server to slow down and try again later, because the reputation of your sending IP address or domain is very low. The message usually looks like this:

421-4.7.0 [203.0.113.10] Our system has detected that this message is
421-4.7.0 suspicious due to the very low reputation of the sending IP address.
421-4.7.0 To protect our users from spam, mail sent from your IP address has
421-4.7.0 been temporarily rate limited. For more information, go to
421 4.7.0 https://support.google.com/mail/answer/188131

A variant says "very low reputation of the sending domain". The code 421 is temporary: your server keeps the email in its queue and retries. If the problem continues for too long (often 1 to 5 days), the server gives up and the sender gets a final bounce.

Why it happens

• Spam complaints. Gmail users clicked "Report spam" on your emails.
• Sending to bad addresses. Old lists with many addresses that no longer exist.
• New IP or domain. No history, and a sudden burst of email.
• Weak authentication. Without SPF, DKIM and DMARC, Gmail cannot build a reputation for your domain properly.
• A compromised account or website on your server sending spam without your knowledge.
• Shared IP. Other customers of your host have damaged the IP you share.

How to check

• Send a test email: on the Delivmail home page (/), copy your unique test address and send it a normal email from the same server, tool or mailbox that bounced. Open the report when it is ready.
• Check authentication and blacklists in the report. A listing on a public blacklist often goes together with a low Gmail reputation.
• Check your IP and domain on /blacklists.
• Open Google Postmaster Tools (postmaster.google.com), add your domain and look at the reputation and spam rate charts. Data only appears if you send enough email to Gmail.
• Look at your mail server queue and logs for unexpected volumes or senders you do not recognise.

How to fix it

1. Stop any unwanted sending. If an account, form or script on your server is sending spam, close it and change passwords first. Nothing else helps while spam is still going out.
2. Pause bulk campaigns to Gmail. Keep only important transactional emails (orders, password resets) for a few days.
3. Set up SPF, DKIM and DMARC with a From domain aligned with SPF or DKIM.
4. Clean your list. Remove addresses that bounced, people who have not opened anything in months, and any address you did not collect with clear consent.
5. Make unsubscribing easy. Add a visible unsubscribe link and the one-click unsubscribe header to marketing emails.
6. Restart slowly. Send first to your most engaged contacts, in small batches, and increase volume over several days or weeks.
7. Do not rotate to new IPs or domains to escape the block. Gmail treats this as evasion, and the new ones will be blocked too.

If you are on shared hosting

If you cannot control the IP, ask your host to investigate, or send through a reputable email service with authenticated sending for your domain.

Check that it is fixed

Send a new Delivmail test to make sure authentication passes and nothing is listed. Then watch your server logs: the 421 deferrals should become less frequent and then stop. Send a normal email to a Gmail address you control. Reputation recovers over days or weeks of clean, wanted email, not instantly, and there is no form to request a faster review.