SMTP Test: Verify SMTP Connectivity
Why SMTP tests matter
SMTP connectivity issues often explain why transactional or marketing emails fail to send. Firewalls, wrong ports, or misconfigured relays can block or delay mail without obvious error messages in your app.
A quick SMTP test verifies that the host responds on the given port and whether STARTTLS is advertised. That tells you if the path is open and if encryption is available.
Before or after changing email providers, run this test along with an MX lookup and SPF checker so you know DNS and connectivity are both correct.
What this tool checks
The tool performs an SMTP handshake: it connects to the host and port, sends EHLO, and inspects the server response. It reports whether the connection succeeded, latency, and whether STARTTLS is supported.
Port 25 is frequently blocked by cloud providers and residential ISPs. Ports 587 (submission) and 465 (SMTPS) are more commonly allowed. If 25 fails, try 587 or 465 and ensure your application is configured to use them.
STARTTLS upgrades the connection to TLS. If the tool reports STARTTLS not supported, check whether you are using the right port or if the server requires implicit TLS (e.g. 465) instead.
How to use it
Enter the SMTP host (e.g. smtp.example.com or the host your provider documents) and the port (587, 465, or 25). Run the test.
If the host is unreachable, check firewall rules, security groups, and whether the service is listening on that port. A port checker can confirm that the port is open from the internet.
If STARTTLS is not supported but you expect it, verify the port and the server's documentation. Some providers use different ports for plain vs. TLS.
Typical issues
Connection timeout: the host is down, the port is wrong, or a firewall is blocking. Confirm the hostname with a DNS lookup and test the port with our port checker.
STARTTLS missing: required for modern deliverability. Switch to a port or server that supports it.
Wrong host or port: double-check your provider's docs. MX lookup shows where mail is received; outbound SMTP often uses a different host (e.g. smtp.provider.com).
Next steps
Once SMTP is reachable, validate SPF and MX so receiving servers accept your mail. Monitoring your mail and web endpoints helps you catch outages before they impact users.