Check for SMTP TLS from command line with OpenSSL Type the following against a mail server to test: $ openssl s_client -connect mail.example.com:25 -starttls smtp For secure SMTP, you can use one of following: openssl s_client -starttls smtp -connect example.com:25 openssl s_client -starttls smtp -connect example.com:465 openssl s_client -starttls smtp -connect example.com:587 250-test.rtcamp.com 250-PIPELINING 250-SIZE 10240000 250-VRFY 250-ETRN 250-STARTTLS openssl to negotiate SSL encryption for openssl s_client -starttls smtp -crlf -connect 127.0.0 I took this command set from an online tutorial and was How To Encrypt OpenLDAP Connections Using STARTTLS Our OpenSSL process can now access the key file properly. Test the STARTTLS upgrade by typing this: I am trying to create an email client as a learning experience. Most modern email servers require usage of TLS. When using OpenSSL's s_client, I can use the -starttls flag so s_client would automat 2 Testing with OpenSSL. I am not saying that you should use OpenSSL for everyday testing; client -connect gmail-smtp-in.l.google.com:25 -starttls smtp. Test STARTTLS configuration of SMTP provide a way to test a SMTP server for STARTTLS server as well as starttls with openssl s_client -starttls How to test SMTP servers using the If you need to test TLS connections you can use the OpenSSL # openssl s_client -connect vsp2.example.local:25-starttls In order to mitigate the "Poodle" vulnerability, I'd like to disable SSLv3 support in my (in this case, TLS, rather than ) server. How can I use openssl s_client to verify that I've done this?