# Error: 421 Connection timeout: network issues

Source: https://catalystgroup.gg/docs/email-delivery/error-421-connection-timeout/
Updated: 2026-10-19 · By Duane Garner

The mail server is temporarily refusing service and closed the connection. Wait and retry; if it keeps happening, slow down or check your server's IP.

> **Shown by SMTP-based sending (Other SMTP, Zoho Mail, the backup SMTP transport):** The test result or the error log shows a reply starting with `421`, for example `421 Service not available, closing transmission channel` or `421 4.7.0 Try again later`.

 

This article is for site owners sending through an SMTP server who get a 421 reply.

 

## What this error means

 

Reply code 421 means the mail server is not able to serve you right now and is closing the connection. The 4 at the start marks it as temporary. The server is not saying your settings are wrong or your message is bad; it is saying "not now". Senders are expected to wait and try again.

 

Mail servers use 421 for several temporary states, so the text after the code is worth reading: it often names the reason.

 

## Common causes

 
- The server is overloaded, under maintenance or restarting.
- You opened too many connections or sent too many messages in a short time, so the server is slowing you down.
- Your server's IP address has a poor or unknown reputation, so the server is deferring you while it decides.
- The session took too long, for example a slow network, and the server timed it out.
- Greylisting: some servers refuse the first attempt from an unfamiliar sender on purpose and accept a retry minutes later.
- A network problem between your web server and the mail server.

 

## How to fix

 
1. Wait and retry
  Give it a few minutes, then go to **CRM-NG SMTP → Tools → Email Test**, fill in **Send To** and press **Send Email**.
2. Read the full reply
  Open the error in **CRM-NG SMTP → Tools → Debug Events**. Words such as "too many connections", "rate" or "reputation" point to the cause.
3. Slow down sending
  If bursts trigger it, go to **CRM-NG SMTP → Settings → Misc** and set **Email Rate Limiting** to stay under your provider's limits. Turn on **Optimize Email Sending** so mail goes out in the background.
4. Check your server's IP
  If the reply mentions reputation or blocking, look up your web server's outbound IP on a public blocklist checker and ask your host to help clear it.
5. Ask your mail provider
  If it continues for hours, contact the provider with the full reply and the time it happened.

 

## Verify it worked

 

The test page shows "Test email sent successfully! Check your inbox to confirm delivery." The Email Log shows no new 421 failures over the following hours.

 

## Notes

 
- A [backup SMTP transport](https://catalystgroup.gg/docs/email-delivery/backup-smtp-transport/) on the **Alerts** tab can carry mail while the main server is deferring you.
- If you send through a shared host's mail server, its limits are often low. A dedicated mail service, through an API mailer, avoids those limits.

 

## Related

 
- [Other SMTP mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-other-smtp/)
- [Zoho Mail mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-zoho/)
- [Misc settings](https://catalystgroup.gg/docs/email-delivery/misc-settings/)
- [Error: 451 Temporary local problem or timeout](https://catalystgroup.gg/docs/email-delivery/error-451-temporary-server-problem/)
- [Error: Connection timed out (code 110)](https://catalystgroup.gg/docs/email-delivery/error-connection-timed-out/)
- [Troubleshooting email delivery](https://catalystgroup.gg/docs/email-delivery/troubleshooting/)