Troubleshoot SMTP Errors

If you're not the administrator of the mail server in question, we suggest you contact your email administrator directly with the error message you're receiving.

If you're seeing an error code in your SMTP logs indicating that the messages you're trying to send are "temporarily deferred," this means that we've observed unusual traffic patterns from your IP address.

Below is a list of common SMTP error codes. For more information, see our SMTP errors list.

  • 421 Message from (x.x.x.x) temporarily deferred - 4.16.50
  • 421 4.16.55 [TS01] Messages from x.x.x.x temporarily deferred due to excessive user complaints 
  • 421 4.16.56 [TS02] Messages from x.x.x.x temporarily deferred due to excessive user complaints 
  • 421 4.7.1 [TS03] All messages from x.x.x.x permanently deferred
  • 451 Resources temporarily not available - Please try again later [#4.16.5]
  • 550 5.7.1 [BLXX] Connections not accepted from IP addresses on Spamhaus
  • 554 5.7.5 (AU01) Message not accepted for policy reasons
  • 554 Message type not allowed
  • "4xx" or "5xx" error message

These typically aren't permanent errors and your system may automatically retry after a while. We encourage you to try re-sending to our emails approximately four hours after you experience the error. If you're seeing this same error consistently over a 48-hour period, we encourage you to review our recommended best practices when sending to Yahoo Mail.