Is the open-relay test safe?

Yes. It stops after RCPT TO with fake addresses and never sends a message.

Why does my server lack STARTTLS?

TLS is not configured or the certificate is broken. Without it mail travels in the clear and some receivers refuse it.

SMTP Diagnostics

SMTP is the protocol mail servers use to hand email to each other. This test connects to a mail server or a domain’s MX and checks its banner, EHLO capabilities, STARTTLS, reverse DNS, blocklist status, and whether it relays mail for outsiders.

When to use this: A mail server rejects connections, TLS fails, or you want to prove a server is not an open relay. Command prefix smtp:

About the SMTP Diagnostics

Connect to a mail server (or a domain’s MX) and check the banner, EHLO capabilities, STARTTLS, reverse DNS, blacklist status and whether it relays mail for outsiders.

FAQ

Is the open-relay test safe?: Yes. It stops after RCPT TO with fake addresses and never sends a message.
Why does my server lack STARTTLS?: TLS is not configured or the certificate is broken. Without it mail travels in the clear and some receivers refuse it.