SMTP is a really simple protocol that you can test by hand! If you have a tool like
netcat handy, you can just type the commands manually. If you are on Windows, you can install the telnet client from the "Install or remove Windows components" menu. If you are on Linux, I would recommend using
netcat -vC (the
v option for verbosity and the
C option for CRLF line endings).
First, open a connection:
nc -vC mail.example.com 25
telnet mail.example.com 25
Then, try sending an email without using encryption:
. on an line by itself, you should get the message that the mail is queued for delivery. Note that you will want to replace
@example.com with your own domain, because the server will almost certainly not allow deliveries from an external domain to an external domain (this would be an open relay).
If the email is accepted, then clearly no encryption is enforced. If it is not allowed, you will get an error message somewhere in this process.
Alternatively, you could use an external service to test it. Search online for "smtp tester" or "starttls tester" and you should find some services. I found three that look promising in the top few results, but they do not explicitly claim to test enforcement of TLS, so I'm not sure if that specific test also exists.