Most people I know use a
telnet to port 25 when they need to test SMTP to a
mail server, be it with Exim, sendmail or any other. I did that for
years, and it is of course quite sufficient and rather effective for basic
connectivity testing, but it gets inefficient quickly. Testing SSL or TLS
cannot be done with
telnet of course, but OpenSSL's
s_client can be
used for that.
If you need to do more, such as testing TLS or even SMTP authentication (SMTP
AUTH), I strongly recommend John Jetmore's swaks. Swaks, the SMTP
transaction tester, is a Perl program which requires a few modules for its
advanced features but performs basic tasks without them. Do read the
documentation (the program is its own manual:
perldoc swaks) for a list of
all its features.