telnet example.com 25

ehlo altest.com

mail from: example@altest.com

rcpt to: al@nudnov.net

data
Subject: My Telnet Test Email
Hello,
This is an email sent by using the telnet command.
Your friend,
Me

.

From:

https://kb.mediatemple.net/questions/889/Sending+or+viewing+emails+using+telnet