SMTP conversation test
Watch a mail server’s opening handshake and what it advertises.
Runs on the server, rate limited
This sends EHLO and QUIT and nothing else. It never sends MAIL FROM or RCPT TO, so it cannot check whether an address exists or probe a server for open relaying. Port 25 is blocked outbound by most hosting providers, so if it times out there, try 587 before concluding the server is down.