Improved SMTP Responses and Throttling Changes

SMTP Responses

The robustness of our SMTP server has been improved by providing better response codes.

  • Response code 553 when email is missing a recipient
  • Response code 421 when inbound email is being throttled and the server is unable to accept the message until a later time
  • Response code 554 with meaningful error message describing the cause (blacklisting or internal server error)

Throttling Improvements

Incoming SMTP connections will be queued based on system load. The connection will stay open while waiting for system resources. If the SMTP connection cannot be completed in a reasonable period of time the server will issue a response code of 421. This situation may be encountered while sending a large amount of email from a specific server or to Mailsac hosted email address.