Just like the testing of SMTP using Telnet, you can also perform the same steps if the target server is using TLS, however because telnet won’t do the job here, you can use OpenSSL instead. Here’s a simple example:
openssl s_client -starttls smtp -crlf -connect mail.domain.com:25
ehlo senderdomain.com
mail from:info@senderdomain.com
rcpt to:info@domain.com
data
To:info@domain.com
From:info@senderdomain.com
Subject:A Test Message for You!
Hello,
How are you?
Thanks
Tristan
.