Troubleshooting SMTP codes and errors

In general terms, SmartRelay is an RFC 5321 compliant message transfer agent (MTA). Your local SMTP forwarder, or alternative client, should be able to manage and respond to standard RFC 5321 behavior, including SMTP reply codes. See SmartRelay overview for more about the local SMTP forwarder.

SMTP code summary

SMTP reply codes can be summarized as follows:

• 2yz (200 – 299)Success codes. The operation was successfully completed.
  The 250 OK code, for example, includes a message identifier string, which you can use to determine the relevant message.
• 4yz (400 – 499)Temporary error codes. Retry after an appropriate delay.
  A local SMTP forwarder queues the messages and retries without your involvement. If you use an alternative client, the appropriate logic is required.
• 5yz (500 – 599)Permanent error codes.
  The operation did not complete successfully and should not be retried. It will most probably generate the same error repeatedly.

SmartRelay specific codes

The following are examples of selected codes that you may see when using SmartRelay. Click the appropriate link for more details about each code.

• 452 Service unavailable, recipients per message limit reached
• 421 Service unavailable, concurrency limit reached
• 421 Reconfiguration in progress
• 501 RCPT TO syntax error
• 550 Relaying denied