SMTP Code: "421 4.4.1 collect: i/o error on connection from <remote server> "

Products

• Messaging Gateway Hardware

Issue/Introduction

You have noticed following error message when attempting to send emails:

421 4.4.1 collect: i/o error on connection from <remote server>

 

This may appear in mail logs or in delivery reports as “Success with Errors Details.”
The issue is typically observed only when sending emails to specific recipient domains.

Environment

SMG 10.8 10.9

Cause

The error indicates that the SMTP session between the sending and receiving servers was unexpectedly interrupted during message transfer.

Common causes include:

• Temporary issues on the recipient mail server (resource overload, service restarts, or disk/network problems).

• Network interruptions during the SMTP session.

• Firewall or security devices dropping/resetting the connection.

When the error is reported for only one destination domain, it is most often related to temporary conditions on the recipient’s server rather than a fault on the sending system.

Resolution

 

• Confirm the scope of the issue

  • Check if the error occurs with all outbound mail or only when sending to a specific domain.

  • If only one domain is affected, it is most likely an issue on the recipient server side.

• Verify message status

  • If the delivery report shows “Success with Errors Details,” the message was accepted by the recipient, but the remote server logged a warning.

  • No immediate action is required in this case.

• Monitor and re-test

  • Retry sending after some time, as the issue is often temporary.

  • If the error persists or messages fail completely (no longer showing “Success”), the recipient domain’s IT team may need to investigate their mail server.

• Optional troubleshooting (if needed)

  • Test connectivity to the recipient domain via telnet <domain> 25 

  • Review firewall/security logs to ensure no SMTP traffic is being dropped locally.

  • Gather log samples for escalation if the issue continues.

 

Additional Information

The SMTP error 421 4.4.1 collect: i/o error generally indicates a temporary interruption on the recipient’s mail server.

If it affects only one domain and the status is “Success with Errors Details,” the message is delivered successfully and no corrective action is required on the sending side.