Perl in a Nutshell, 2nd Edition

The SMTP Protocol and the SMTP Session

The SMTP protocol defines the set of commands a client sends to an SMTP server, which is generally bound to port 25 of a mailhost. Requests and responses are negotiated between client and server.

When a client negotiates an SMTP session with a server, the server tells the client that it’s listening. Once you’re connected, you introduce yourself to the server by issuing a HELO command. The HELO command accepts one parameter—your hostname—and defaults to your remote hostname if you don’t specify one. If the command is successful, the server sends a 250 response, as follows:

HELO
250 mail.somename.com Hello some-remote-host.com [127.0.0.1], pleased to meet 
you

After you’ve been greeted by the server, send the MAIL command to tell the server who the message is from. The MAIL command takes the string From: user@hostname as an argument, and the server responds with a 250 message to indicate success:

MAIL From: <[e-mail]>
250 [e-mail] ... Sender ok

Then send the RCPT command to tell the server who the recipient is:

RCPT To: <[e-mail]>
250 [e-mail] ... Recipient ok

Now you’re ready to send the body of your message to the server. The DATA command tells the server that all data until a . on a line by itself should be treated as the body of the mail message:

DATA 354 Enter mail, end with "." on a line by itself Subject: Hi, just thought you'd be interested ... Hi, this is the text of my mail message that ...