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TCP connect test against common or custom ports.
The Port Check tool performs a TCP connect test against one or more ports on a target host to confirm whether a service is listening and accepting connections from the public internet.
Enter a host and port in the form host:port (for example mail.example.org:25). You can also test several comma-separated ports. Run the tool to probe each port from our edge location.
Input: Host and port — e.g. mail.example.org:25.
Each result line lists the port number, protocol, and whether it is OPEN, CLOSED or FILTERED. OPEN means a service accepted the TCP connection; CLOSED means the host refused it (RST); FILTERED means no response was received, typically due to a firewall dropping the packet.
→ Confirm a web (80/443) or mail (25/465/587) service is reachable externally.
→ Verify a firewall change opened or closed the intended port.
→ Check that only expected ports are exposed on a new server.
→ Diagnose why an external client cannot reach an internal service.
A closed port returns an explicit TCP RST, meaning the host is up but no service listens there. A filtered port returns nothing at all, meaning a firewall or ACL is dropping the packet — you cannot tell whether a service is behind it.
A TCP connect test against hosts you own or have permission to test is a normal administrative action. Scanning third-party systems without authorisation may violate acceptable-use policies or local law; only test infrastructure you control or are authorised to assess.
Test 25 (SMTP), 465 (SMTPS), 587 (submission) and 993 (IMAPS)/995 (POP3S). If 25 is filtered from the public internet, other mail servers cannot deliver inbound mail to you.