SMTP commands and reply codes across a TCP/IP connection

When SMTP commands arrive over a TCP/IP connection, all responses (positive or negative) are returned over that TCP/IP connection. See RFC 2821 for more detailed information about the SMTP protocol and the SMTP commands and reply code sequences that are generated between an SMTP client and SMTP server.

The SMTP reply code is generated by the remote SMTP server when it receives an SMTP command over a TCP/IP connection. These reply codes might appear in the CSSMTP log file or in error reports. All SMTP reply codes contain a 3-digit number. The first digit of the reply code indicates the successful responses shown in the following table:

First digit	Description
2	Indicates a positive response; the command was accepted.
3	Indicates a positive response to the SMTP DATA command. The remote SMTP server is ready to receive the data that is associated with the command.
4	Indicates a temporary negative response. Reissue this command at a later time.
5	Indicates a permanent negative response. The command has been rejected.