Gmail Is Delaying Your Email With 421 4.7.0? Act Before It Bounces

The fix for a Gmail 421 4.7.0 deferral is to make your mail look trustworthy before the retries run out. Authenticate it with SPF, DKIM and DMARC, send over TLS, give your sending IP a reverse DNS record, and slow down if you're sending a burst. A 421 is a "not now," not a "never," but if the cause stays, the delay turns into a bounce.

What does Gmail 421 4.7.0 mean?

Gmail 421 4.7.0 means Gmail temporarily refused your message and closed the connection. The 4 at the front marks a temporary failure under the SMTP status code standard, RFC 3463, so your mail server holds the message and tries again later. Google's Gmail SMTP error reference lists several 421 4.7.0 messages, including:

• "This message is suspicious due to the very low reputation of the sending IP address."
• "This message is suspicious due to the very low reputation of the sending domain."
• "TLS required for RCPT domain, closing connection."
• A version that says the sending IP "does not have a PTR record."

You usually see a 421 as a "delayed" warning first. If your server keeps getting it, the warning turns into a failed delivery notice days later.

What does a Gmail 421 delay mean for my business?

A Gmail 421 delay means customers on Gmail or Google Workspace get your email late, or not at all if the delay outlasts your server's retries. Time-sensitive mail hurts most: appointment reminders, quotes and password resets. Gmail is telling you it isn't sure your mail is really from you, or that you're sending more than it expects.

What do the 421 authentication codes mean?

These codes mean Gmail is rate limiting your mail because it failed authentication. Google's error reference words them this way:

• 4.7.26: "This email has been rate limited because it is unauthenticated."
• 4.7.27: rate limited "because SPF authentication didn't pass for this message."
• 4.7.30: rate limited "because DKIM authentication didn't pass for this message."
• 4.7.32: rate limited because the From header "isn't aligned" with the authenticated domain.
• 4.7.40: rate limited "because the sending domain doesn't have a DMARC record."

Each one names the record to fix. If you also see 550 5.7.26, the same failure has gone permanent. The Gmail 550 5.7.26 guide covers that version.

What does 421 4.7.28 mean?

Gmail 421 4.7.28 means Gmail "has detected an unusual rate of email" from your IP address, IP block, DKIM domain, SPF domain, or a link domain in your messages. Google's email sender guidelines say "When you exceed your quota, Gmail typically rejects messages with SMTP error code 4.7.28." In plain terms, you sent too much too fast, or enough people marked it as spam that Gmail slowed you down. A newsletter blast from a new tool, or a hacked mailbox sending spam, are classic causes.

How do I fix Gmail 421 deferrals step by step?

You fix Gmail 421 deferrals by fixing the reason in the message text, then giving your reputation time to recover. Google's sender guidelines set the baseline every sender has to meet. In order:

1. Read the full line. The words after 421 4.7.x tell you whether it's authentication, reputation, TLS, reverse DNS or volume.
2. Authenticate. Set up SPF and DKIM for every service that sends as you, and publish DMARC. Check where you stand with the free email security check.
3. Use TLS. Google's guidelines say to "Use a TLS connection for transmitting email."
4. Fix reverse DNS. Google's guidelines say to "Ensure that sending domains or IPs have valid forward and reverse DNS records." The 550 5.7.25 reverse DNS guide shows how.
5. Slow down. Spread big sends out over hours instead of minutes, and send only to people who asked for your mail.
6. Watch your spam rate. Google says to keep spam rates in Postmaster Tools "below 0.10% and avoid ever reaching a spam rate of 0.30% or higher."
7. Check for a hacked account. A sudden 4.7.28 from a business that doesn't send bulk mail can mean someone else is sending as you.

To check a delay notice, paste it into the free reader on the 421 4.7.0 low reputation page at postmortem.report. It runs in your browser and names which 421 you have. Got a different code? The email rejection error codes guide lists every code with its own fix page.

Does Microsoft send a similar temporary error?

Yes. Microsoft 365 has its own temporary block, 4.7.500 through 4.7.699, "Access denied, please try again later," according to Microsoft's NDR reference. Microsoft says "Suspicious activity was detected and sending was temporarily restricted," and that a valid sender's restriction "is lifted shortly." The same cleanup helps with both providers.

When should you get help with Gmail deferrals?

You should get help when the deferrals keep coming after authentication is in place, or when you can't tell which service is sending the mail Gmail flags. If you run a business in Battle Creek, Kalamazoo or anywhere in Southwest Michigan, the Parallax DMARC audit service finds every sender on your domain, sets up SPF, DKIM and DMARC, and reads the reports so a problem sender shows up early. Call (269) 460-1772 or use the contact form and tell us what the notice says.