SMTP Error 421: Service Not Available - Try Again Later

Temporary failure Medium severity Network RFC 5321

What it means

SMTP Error 421 means “Service Not Available - Try Again Later.” The receiving mail server is temporarily unable to process your email and has asked you to retry later. This soft bounce is commonly caused by rate limiting, server overload, greylisting, or temporary IP reputation issues with providers like Gmail and Microsoft.

At a glance

Code421

Bounce typeSoft (temporary)

SeverityMedium

CategoryNetwork

What to doQueued and retried automatically

StandardRFC 5321

What it looks like in your mail logs

421-4.7.28 [203.0.113.10] Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited. - gsmtp

What does 421 mean?

SMTP code 421 is one of the most common soft bounce codes and indicates a temporary service unavailability. The receiving server is telling your sending server to disconnect and try again later. Unlike 5xx permanent failures, a 421 response means the email may succeed on a subsequent attempt.

Gmail frequently returns 421 4.7.0 or 421 4.7.28 when it detects suspicious sending patterns, high volume from a new IP, or temporary reputation concerns. Microsoft 365 uses 421 4.7.0 for throttling and greylisting. Yahoo returns 421 with codes like TS01 or TS02 for rate limiting based on IP reputation. Most properly configured sending servers will automatically retry after receiving a 421.

Repeated 421 responses from the same provider are a warning sign. If your server consistently gets 421 from Gmail or Microsoft, it usually indicates a reputation problem that needs to be addressed - not just a temporary server issue. Check your IP and domain reputation, review your sending patterns, and ensure you are properly warming up new IPs.

How 421 plays out

Your server attempts delivery

The recipient defers with a temporary 421 reply

Your server queues the message and retries on a back-off schedule

It delivers on a later attempt, or becomes a hard bounce if it keeps failing

Where 421 sits: soft vs hard bounce

	Soft bounce (4xx)	Hard bounce (5xx)
Nature	Temporary	Permanent
SMTP class	`4xx`	`5xx`
What to do	Let it retry	Suppress the address
Recoverable?	Often	No
421 is	✓ this code

Common causes of 421

Rate limiting - you are sending too many emails too quickly to this provider
IP reputation issue - the receiving server does not trust your sending IP
Greylisting - server temporarily rejects first-time senders to filter spam
Server overload or maintenance on the receiving end
New IP without established sending reputation (cold IP)
Sudden spike in sending volume triggering throttling

How to fix 421

Wait and retry automatically - most MTAs handle this. Check your retry queue settings
Reduce sending rate to the affected provider (implement throttling)
Warm up new IPs gradually - start with 50-100 emails/day and increase over 2-4 weeks
Check your IP reputation using Sender Reputation Checker and blacklist tools
Spread sending volume across multiple IPs if sending at high volume
Review Google Postmaster Tools or Microsoft SNDS for reputation data from those providers

Frequently asked questions

What does SMTP error 421 mean?

SMTP error 421 means the recipient's mail server is temporarily unavailable and cannot process your message right now. This is a "soft bounce" or transient error, indicated by its 4xx-class status code, and the server is essentially asking your mail system to try again later. Unlike permanent 5xx errors, a 421 error typically resolves on its own as the receiving server recovers from high traffic, rate limiting, or temporary maintenance.

Will my email be automatically retried after a 421 error?

Yes, your mail server should automatically retry delivery after receiving a 421 error. Most mail servers use an exponential backoff mechanism, where the first retry occurs after a few minutes and subsequent retries use progressively longer intervals. If retries continue to fail over a period of several days (typically 4-5 days), the message will eventually generate a permanent bounce notification back to you.

What causes Gmail to return a 421 4.7.0 error?

Gmail returns a 421 4.7.0 error when your sending IP or domain has exceeded Gmail's rate limits for connections or messages within a specific timeframe. Google imposes strict sending thresholds to prevent abuse, and exceeding them triggers temporary rate limiting. To resolve this, immediately reduce your sending volume, add delays between messages, pause bulk campaigns, and implement exponential backoff in your sending infrastructure.

How do I fix SMTP error 421 "service not available"?

To fix a 421 "service not available" error, start by simply waiting 15-30 minutes before retrying, since the issue is usually temporary. If the error persists, check whether you are exceeding the recipient server's rate limits by sending too many emails too quickly. Verify your DNS and MX records are correctly configured, and ensure your sending server is not experiencing network connectivity issues. If you repeatedly get 421 errors from a specific provider like Gmail or Microsoft, reduce your sending speed to that domain.

What is the difference between a 421 and a 550 SMTP error?

The key difference is that a 421 error is temporary (soft bounce) while a 550 error is permanent (hard bounce). A 421 error means the server cannot accept mail right now but may accept it later, and your mail server will automatically retry delivery. A 550 error means the server has permanently rejected the message and will not accept retries; you must fix the underlying problem first. Do not remove addresses from your list based solely on 421 errors, as they typically resolve on their own.

Related bounce codes

Sources & further reading

RFC 5321 · The standard that defines this reply code Gmail bulk sender guidelines · Google sending limits and the rate limiting that triggers 421 4.7.28 deferrals Microsoft 365 SMTP throttling · Concurrent connection and storage throttling behind Exchange Online 421 4.3.2 deferrals

Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes