Posted by mikeb on March 26, 2013
One issue that comes up often in testing SMTP connections is in regards to TLS.
When we need to do this it adds a layer of complexity that has some think it cannot be done. This is the greatness of openssl and below is what you’ll need (bare bones of course) to manually test:
openssl s_client –connect <recipient MTA>:<port> -starttls smtp
You will also need to provide information concerning the TLS certificate exchange, you’ll get a prompt to type the remaining SMTP commands like normal (e.g. MAIL FROM, RCPT TO:, etc). You can also add specific paths to reference a custom TLS certificate location.