On this page
Short answer
421 4.7.0 is a temporary refusal: Gmail deferred the email and your server will retry. Google uses the same code for several causes, so read the words after it. For cold email the common ones are very low reputation of the sending IP or domain, a sending IP without a matching PTR record, and suspicious content or links. Lower volume at once, then fix the cause the text names.
Eight texts share the code
Google's SMTP error reference (updated October 13, 2026) lists eight 421 4.7.0 replies. They have little in common beyond being temporary:
Text after 421 4.7.0 | Cause | What to do |
|---|---|---|
"very low reputation of the sending IP address ... temporarily rate limited" | IP reputation | Cut volume from that IP; ask your provider about it |
"very low reputation of the sending domain ... the message has been blocked" | Domain reputation | Stop cold email from the domain for now |
"suspicious due to the nature of the content or the links within" | Content or a linked domain | Remove links and tracking, rewrite the offer |
"does not have a PTR record, or the corresponding forward DNS entry does not point to the sending IP" | Reverse DNS on the sending server | The server's owner sets a matching PTR |
"TLS required for RCPT domain, closing connection" | The recipient's admin requires TLS | Turn on STARTTLS on the sending server |
"IP not in whitelist for RCPT domain, closing connection" | The recipient accepts mail only from listed IPs | Remove the recipient |
"Try again later, closing connection" | Generic deferral | Let it retry; watch for a pattern |
"Connection expired, try reconnecting" | Network timeout | Nothing; the retry delivers |
Paste the full bounce into the email bounce decoder to match it automatically.
The reputation variants
Two of the eight matter most for cold senders. The IP version says Gmail has "temporarily rate limited" email from that IP; the domain version says the message "has been blocked". Both mean Gmail's view of your sending is poor right now, and both get worse if you keep pushing the same volume.
Cut volume immediately on the affected mailboxes, to a level Gmail accepts without deferrals.
Find which one it is. If mailboxes on other domains but the same server also get the IP text, the server is the problem. If one domain gets it on every server, the domain is.
Check the spam rate in Google Postmaster Tools. Google's sender guidelines require it to stay below 0.3%.
Fix list and copy. Bounced and unengaged recipients and link-heavy first emails push reputation down. Run the copy through the cold email copy checker.
Restart low with warmup running, and raise volume in steps over one to two weeks.
A domain that keeps getting the domain-reputation text after a pause is usually not worth saving for cold email. See burned domain: what to do.
Free guide19 min read
Send 100,000 cold emails a month
Domains and mailboxes you need, DNS, warmup, lists, copy and follow-ups, with a launch checklist, sent to your inbox.
421 4.7.0 compared with 421 4.7.28 and 550 5.7.1
421 4.7.28 is the rate code: "an unusual rate of unsolicited email" from your IP, domain or a URL. Same treatment, slower volume. See 421 4.7.28.
550 5.7.1 is the permanent version of the reputation texts: "very low reputation of the sending IP address" or "of the sending domain" with a 550 code means Gmail stopped deferring and now rejects. See 550 5.7.1.
If 421 4.7.0 turns into 550 5.7.1 for the same mailboxes, the window for slowing down has passed.
What we recommend at Outreach2day
Send up to 25 cold emails per mailbox a day, keep warmup running while you send, and spread volume across mailboxes and domains. That keeps each sender's pattern steady, which is what Gmail's rate and reputation checks reward.
Sources
- Google Workspace Admin Help: Gmail SMTP errors and codes: support.google.com/a/answer/3726730
- Google: Email sender guidelines: support.google.com/a/answer/81126
- Gmail Help: Postmaster Tools dashboards: support.google.com/mail/answer/14668346
Related answers
- What Does Gmail's 421 4.7.28 Error Mean, and How Do I Fix It?Gmail and Google
- What Does a 550 5.7.1 Bounce Mean, and How Do I Fix It?Bounces and errors
- Why Are My Cold Emails Going to Spam in Gmail in 2026?Gmail and Google
- My Cold Email Domain Is Burned. What Should I Do?Domains
In depth on the blog
- Google Postmaster Tools: Setup and Dashboards Guide8 min read
- Google Workspace and Gmail Sending Limits12 min read
