HELO
MAIL FROM: [email protected]
RCPT TO: [email protected]
DATA
test 
test
.

http://www.networkclue.com/internet/SMTP/manual-test.aspx

Mail might not send from user different from current domain or to user in ouside domain.   
Data section terminated by CRLF.CRLF

Manually Send an SMTP Message from a Telnet Session.

C#