Skip to content
iBlockSpam
Get started
Check your domain Get started Log in
How it works
Platforms
Microsoft 365 Google Workspace cPanel / WHM Exchange Server
Solutions
Small business MSPs Hosting providers Law firms Accounting firms Healthcare practices Real estate & title
Free tools Pricing
Blog
All guides
By topic
Email security basics Phishing & threats SPF, DKIM & DMARC Setup guides Running email day to day Switching & migration Choosing a filter
Reference
Glossary Research: top 1,000 sites
SMTP error code · Standard (RFC)

SMTP 421: service not available, closing connection

421 Temporary failure (4xx): the sender retries
By iBlockSpam Team Checked against sources 3 sources How we check our content
What it means

The receiving server is temporarily unable or unwilling to take mail and is closing the connection. Your server should keep the message and retry later.

What the bounce says
421 4.3.0 Temporary System Problem. Try again later.

One of several 421 replies listed in Google's Gmail SMTP errors and codes reference.

What 421 means

RFC 5321 defines 421 as "Service not available, closing transmission channel" and notes that a server may send it in reply to any command when it knows it must shut down. Because it starts with a 4, it is a temporary failure: the message has not bounced. Your outgoing server keeps it in its queue and tries again.

Receivers use 421 for more than maintenance. Gmail, for example, documents 421 replies for a busy server, for an expired connection, for TLS being required by the recipient's domain, and for rate limiting senders with a missing PTR record or low reputation. The enhanced code and text after the 421 tell you which one you hit, so read the whole line.

A single 421 is normal and usually clears on the next attempt. RFC 5321 says senders should wait at least 30 minutes between retries and keep trying for at least four to five days before giving up. If every attempt for hours gets a 421 that mentions rate limits, reputation or authentication, the receiver is deferring you on purpose, and retrying alone won't fix it.

Likely causes

  • The receiving server is overloaded, restarting or under maintenance.
  • The receiver is rate limiting your sending IP address or domain.
  • Your server's IP address has no valid reverse DNS (PTR) record, or a low reputation.
  • The recipient's domain requires TLS and your server didn't use it.
  • Greylisting: the receiver defers unknown senders once and accepts the retry.

How to fix it

If you sent the message

  1. Let your server retry. Don't resend by hand, which only adds to the queue.
  2. If the 421 repeats for hours, read the enhanced code and text, then fix the cause it names (PTR, authentication, TLS or sending volume).
  3. Check your sending IP with a reverse DNS lookup and a blocklist check.
  4. Spread large sends over time instead of sending in one burst.

If you're the recipient or their admin

  1. If your own server sends 421 to everyone, check its load, disk space and service status.
  2. Review rate limits and greylisting settings if partners report long delays.
Reverse DNS (PTR) Lookup Blacklist Checker Email Health Check

Frequently asked questions

Will a message that got a 421 still be delivered?

Usually, yes. A 421 is a temporary failure, so the sending server keeps the message and retries. It only becomes a bounce if every retry fails until the sender's queue lifetime runs out, which RFC 5321 suggests should be at least four to five days.

Should I resend the email myself after a 421?

No. Your mail server is already retrying. Sending another copy adds load and can make rate limiting worse. If you received a final bounce after days of 421 replies, fix the cause named in the error first, then resend.

Why does Gmail send 421 errors to my server?

Gmail uses 421 replies to rate limit senders it doesn't trust yet, for example when the sending IP has no PTR record, has a low reputation, or sends at an unusual rate. The text after the code names the reason and links to Google's sender guidelines.

Sources

  1. RFC 5321: Simple Mail Transfer Protocol (reply codes, section 4.2)
  2. RFC 5321, section 4.5.4.1: Sending strategy and retry intervals
  3. Gmail SMTP errors and codes (Google Workspace Admin Help)

Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.

Related codes

Related guides

Running email day to day

What happens to email when your mail server is down?

Senders retry when your mail server is down, but not forever. How SMTP retries work, 4xx vs 5xx errors, what senders see, and how outage spooling helps.

8 min read Running email day to day

Reverse DNS and PTR records for mail servers

What a PTR record is, why Gmail, Yahoo and others require forward-confirmed reverse DNS for sending IPs, who can set it, and how to check yours is right.

8 min read SPF, DKIM & DMARC

Gmail, Yahoo and Microsoft bulk sender requirements explained

What Gmail, Yahoo and Outlook.com require from bulk senders: SPF, DKIM, DMARC, alignment, one-click unsubscribe and spam rates, with the real dates and limits.

9 min read