As part of the server-to-server email delivery dialog (SMTP), a three-digit enhanced SMTP response code is provided that quantifies the disposition of the email message. These status codes are often very useful for understanding specifically “why” a message was not delivered, without needing to resort to parsing varied plain-text descriptive reasons.
These Enhanced Status Codes are defined in RFC 3463 are consist of three digits. The first digit identifies the “class” of the SMTP status code:
The second digit identifies the “subject” of the SMTP status code:
The last digit provides more detail on the response category.
The definitions of the commonly used Enhanced SMTP Status Codes are well defined by the RFC. We refer interested parties to the specification for an accurate exposition of these meanings.
In addition to the common Enhanced SMTP Status Codes, LuxSci reserves some additional 3-digit status codes for LuxSci-specific uses. These are as follows: