SMTP Server Test
Enter a mail server hostname (or a domain — we'll test its primary MX) and SysGlass opens a live SMTP session: it connects, reads the ESMTP capabilities, upgrades the connection with STARTTLS (recording the TLS version), checks the server's reverse DNS, and runs a safe open-relay probe (it offers a relay to an external domain but never sends a message). It's the quick way to confirm a mail server is reachable, encrypted, and not an open relay.
Reviewed by the SysGlass team · Updated July 21, 2026
What it checks
- Whether the server accepts an SMTP connection on port 25 (and how fast)
- If a domain is entered, it resolves and tests the primary MX
- The advertised ESMTP capabilities (STARTTLS, PIPELINING, SIZE, 8BITMIME, AUTH, …)
- STARTTLS support and the negotiated TLS version
- The sending IP's reverse DNS (PTR)
- A safe open-relay test (RCPT to an external domain — no message is sent)
How to read the result
- OK = connects, offers STARTTLS, and refuses to relay to external domains.
- Warn (no STARTTLS) = mail to this server is delivered in plaintext; enable STARTTLS.
- Fail (open relay) = the server relayed to an external domain — fix immediately; it will be abused and blacklisted.
- Could not connect = outbound port 25 may be blocked, or the server is down.
Frequently asked questions
What is an open relay and why is it bad?
An open relay accepts mail for external domains from anyone, so spammers use it to send through your server. It gets blacklisted quickly and ruins your IP reputation. A correct server only accepts mail for its own domains or from authenticated senders.
Does the open-relay test actually send email?
No. It issues MAIL FROM and RCPT TO for an external test address and checks whether the server accepts the recipient, then resets the session. No DATA (no message) is ever sent.
Can I test a domain instead of a specific server?
Yes — enter a domain and SysGlass looks up its MX records and tests the primary MX, which is what other mail servers connect to.
Why does it say it couldn't connect?
Many networks block outbound port 25 to fight spam. SysGlass runs the test from a vantage point that allows port 25; if the target itself is down or firewalled, the connection will still fail.