1. To send email (usually port 25, -C is to send a complete line feed to imitate telnet):
   nc -C IP PORT
2. Then use the following line by line:
   EHLO domainname (or HELO)
MAIL FROM: a@email.com
RCPT TO: recepient@email.com
DATA
3. From here can enter the email content, until a single dot. Usually can put something like below:
   From: bar@example.org
To: foo@example.com
Subject: Test
Date: Thu, 20 Dec 2012 12:00:00 +0000

Content
.

QUIT to end