A set of Simple Mail Transfer Protocol (SMTP) commands to send an email:

HELO somehostname
MAIL FROM:fromaddress@host1.tld
RCPT TO:toaddress@host2.tld
DATA
To: "To Address" <toaddress@host2.tld>
From: "From Address" <fromaddress@host1.tld>
Subject: An Email
This is content.
 
Here is another line.
.
QUIT

Note that MAIL FROM / From and RCPT TO / To are not actually required to match, though failure to fill in the MAIL FROM / RCPT TO commands may result in the message being rejected. The commands above are not case-sensitive, and the message ends with a . on a single line.