I've outlined here a short example of using telnet to test SMTP services on a server.

telnet mail.server.domain 25
Trying ???.???.???.???...
Connected to mail.server.domain.
Escape character is '^]'.
220 mail.server.domain ESMTP Sendmail ?version-number?; ?date+time+gmtoffset?

HELO local.domain.name
250 mail.server.domain Hello local.domain.name [loc.al.i.p], pleased to meet you
MAIL FROM: <[email protected]>
250 2.1.0 [email protected]... Sender ok
RCPT TO: <[email protected]>
250 2.1.0 [email protected]... Recipient ok
Subject:-type subject here-

Hello, World!

250 2.0.0 ???????? Message accepted for delivery
221 2.0.0 mail.domain.ext closing connection
Connection closed by foreign host.