The SMTP protocol extension command EHLO

The original SMTP protocol from 1982 requires client application to greet the server with the HELO command. This command should be the first command after the server's greeting line is received.

For example, if I run the following command:

telnet mail.dlc.fi 25

...then the server might respond with:

220 mail.dlc.fi SMTP service ready

For older email servers, the only supported command might be HELO, but newer ones might support the extended hello command EHLO.

There is a new RFC document from last November defines the EHLO command as a service extension. If the email server supports this command, you will get a success code response, such as 250. But if not, the error code is likely 500 (syntax error) or 502 (command no implemented).

The only way to know seems to be to test. It is easy with Telnet, or you could write a simple TCP/IP socket application with Delphi or C.

Happy emailing!