SMTP error code

SMTP error 421: service not available, try again later

SMTP error 421 is a temporary refusal: the receiving server closes the connection and expects your server to come back later. Most 421s clear up on their own. The ones that don't point to your sending reputation or setup.

Updated October 9, 2026 · 4 min read

Reply code
421 Service not available, closing transmission channel (RFC 5321)
Class
Temporary failure
Bounce type
Soft bounce, if it never gets through
Retry?
Yes, your server retries on its own

Verification would not have caught it

A 421 is about the receiving server's state or your sending behavior, not about the address. Verification can't predict it; your server's retries handle most cases.

SMTP error 421 4.4.5 Server busy: the sending server keeps the message in its queue and retries until it gets 250 OK

What SMTP error 421 means

RFC 5321 defines SMTP error 421 as “<domain> Service not available, closing transmission channel”, and adds: “This may be a reply to any command if the service knows it must shut down.” Two things set it apart from other temporary errors. It can come as the answer to any command, not just to a particular recipient or message. And the receiving server hangs up after sending it.

Your mail server treats a 421 as a deferral. It keeps the message in its queue and tries again; RFC 5321 (section 4.5.4.1) says “the retry interval SHOULD be at least 30 minutes” and that “the give-up time generally needs to be at least 4-5 days.” Only if every attempt fails until then do you get a bounce. The guide to hard vs. soft bounces explains how email services count that.

421 error messages by provider

Large providers put the actual reason after the code. Gmail uses 421 a lot; these are excerpts from Google’s SMTP error list:

Gmail replyReason
421 4.3.0 Temporary System Problem. Try again later.A problem on Google’s side
421 4.4.5 Server busy, try again later.Load on Google’s side
421 4.7.0 TLS required for RCPT domain, closing connection.The recipient’s domain requires an encrypted connection
421 4.7.0 The IP address sending this message does not have a PTR record …Reverse DNS for your IP is missing or doesn’t match
421 4.7.0 This message is suspicious due to the very low reputation of the sending IP address.Reputation (other texts name the domain, or the content and links)
421 4.7.26 This email has been rate limited because it is unauthenticated.Neither SPF nor DKIM passed
421 4.7.27, 4.7.30, 4.7.40Rate limited because SPF failed, DKIM failed, or the domain has no DMARC record
421 4.7.28 Gmail has detected an unusual rate of email …Too much mail, or too much unwanted mail, from your IP or domain
421 4.7.29 Your email has been rate limited because you're not using a TLS connection.No TLS

Yahoo’s sender help treats 421 and 451 together as “a temporary problem blocking the delivery of your message”. Its list of causes: unusual traffic from your IP, content that looks like spam, complaints from Yahoo users, busy servers, a name server that can’t be reached, or a failed authentication lookup.

What causes a 421 error

How to fix SMTP error 421 as the sender

  1. Let the retries run. Don’t resend by hand: when the original goes through later, the recipient gets it twice.
  2. If it lasts more than a few hours, read the enhanced code. 4.3.x and 4.4.x are about the receiver. 4.7.x is about you.
  3. Set up reverse DNS. Ask whoever runs your server’s IP address to set a PTR record that matches the server’s host name.
  4. Turn on TLS for outgoing connections on your mail server.
  5. Fix authentication. Publish SPF with the SPF record generator, turn on DKIM signing with your own domain, and add a record with the DMARC generator.
  6. Slow down and clean up. Spread large campaigns over time, warm up new IP addresses gradually, and stop mailing people who never engage. The email deliverability guide covers the long-term work.

If you send through an email service, it handles retries. Its logs show the deferrals; act when one domain keeps deferring you for days.

If you run the receiving server

A 421 from your own server is fine during maintenance or overload. If legitimate senders report long delays, check your rate limits and greylisting setup: an allowlist for known partners avoids delaying their mail. Remember that RFC 6647 notes some clients never retry, so their mail is lost on the first temporary refusal.

Would email verification have prevented it?

No. A 421 says nothing about whether the mailbox exists, and the same address can be deferred today and accept mail tomorrow. Verification helps with one cause behind rate limits, though: sending to many dead addresses. Yahoo lists “excessive unknown recipients” as one of its error categories. Checking a list with the bulk email verifier before a campaign keeps that number low.

Stop bounces before they happen

Most hard bounces come from addresses that don't exist. An email verification asks the receiving server about the mailbox without sending anything, so you can remove bad addresses before your next send.

Check one address Clean a whole list

Frequently asked questions

What does SMTP error 421 mean?

The receiving server can't or won't serve you right now and is closing the connection. It is a temporary error: your mail server keeps the message and tries again later. The enhanced code and text after 421 say why, for example a busy server, a rate limit or missing authentication.

Is a 421 error a bounce?

Not at first. A 421 defers the message. It only turns into a bounce, a soft bounce, if the problem lasts until your server gives up retrying.

How long does my server retry after a 421?

That depends on its settings. RFC 5321 recommends waiting at least 30 minutes between attempts and keeping the message for at least 4 to 5 days before giving up. Email services use their own schedules.

How do I fix 421 4.7.0 at Gmail?

Read the text after the code. Gmail uses 421 4.7.0 for a missing or mismatched PTR record, for connections without TLS where TLS is required, and for a very low IP or domain reputation. Fix reverse DNS and TLS on your server, and slow down if reputation is the reason.

Related codes and guides

SMTP error 451SMTP error 451 is a temporary error on the receiving side, and your server will retry. See what Gmail, Microsoft 365 and Yahoo mean by 451 and when to step in. 550 5.7.1550 5.7.1 means the receiving server refused your email because of a policy. See what Gmail and Microsoft 365 say, the usual causes, and how to fix it. 550 5.4.1550 5.4.1 Recipient address rejected: Access denied is Microsoft 365 refusing an unknown address. See both variants, their causes and the fix for each side. 550 5.1.1550 5.1.1 means the recipient's mailbox doesn't exist. See what Gmail, Microsoft 365 and Postfix say, why it happens, and how to stop these hard bounces. Hard bounce vs. soft bounceHard bounce vs. soft bounce: what each one means, the SMTP codes behind them, how email services handle them, and how to prevent hard bounces before you send. Email deliverabilityEmail deliverability explained: authentication, list quality, engagement, the Gmail, Yahoo and Microsoft sender rules, monitoring tools and a full checklist.

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