About the SMTP Tester
The SMTP test connects to a mail server the same way another server would: it reads the greeting banner, sends EHLO, upgrades the connection with STARTTLS where offered and inspects the TLS certificate. It never logs in and never sends a message.
Use it to confirm a server is reachable, that TLS works with a valid certificate and that the host name in the banner matches its reverse DNS.
What does the SMTP test do?
It connects to the server, reads the greeting, sends EHLO and upgrades to TLS where offered — exactly like another mail server would — and reports timings and certificate details. It never logs in and never sends a message.