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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)

SMTP Commands 1 2 3 4 TCP/IP Electronic Mail Access and Retrieval Protocols and Methods

SMTP Multiple-Line Text Replies

Again, as in FTP, it is possible for a reply to contain more than one line of text. In this case, each line starts with the reply code, and all lines but the last have a hyphen between the reply code and the reply text, to indicate that the reply continues. The last line has a space between the reply code and text, just like a single-line reply. An example of this was given in the topic on SMTP connections where I showed a multi-line response to a EHLO command.

When the ENHANCEDSTATUSCODES SMTP extension is enabled, this causes supplemental reply codes to be issued by the SMTP receiver in response to each command. These codes are similar in some respects to the standard reply codes; they also use three digits, but the digits are separated by periods. These enhanced codes provide more information about the results of operations, especially errors.

For example, if you try to issue a RCPT command specifying a non-local mailbox on a server not supporting this feature, it will send back a “550” reply, which is a generic error (“requested action not taken: mailbox unavailable”). When enhanced status codes are active, the response will be “550 5.7.1”, the more specific message “delivery not authorized, request refused”. A full description of these enhanced codes can be found in RFC 1893.

SMTP Commands 1 2 3 4 TCP/IP Electronic Mail Access and Retrieval Protocols and Methods