Command What we are doing
telnet 25 telnet to a machine that responds to smtp on port 25
HELO issue the 'HELO' command to start (this is a "hello, I am" introduction)
mail issue the 'mail from' command
rcpt issue the 'rcpt to' command (this should be a valid address)
data now the 'data' command will allow you to do the rest...
subject:This is the subject the text you want on the subject line
  a blank line to tell indicate the next part is the message body
This is the message body! this is the message body
. a 'dot' as the first and only character tells the SMTP Service to process the message
quit the 'quit' command exits your telnet on port 25

I typed the stuff in blue below.
You are looking for the return codes in green below, the text returned may change from server to server, but the codes should not.
B:\>telnet 25
220 miniRelay Server v0.9.75 ready
250 Hello []
250 Address Okay
250 Address Okay
354 Start mail input; end with <CRLF>.<CRLF>
subject:This is the subject

This is the message body!
250 Ok, Mail queued for delivery.
221 Signing Off.