Free SMTP Server Test

01 · Definition

What is smtp server test?

An SMTP server test connects to a mail server on port 25, reads the SMTP banner the server returns, and reports which SMTP capabilities it advertises — including STARTTLS, authentication methods and message-size limits. It confirms the server is reachable and responding to SMTP protocol commands without sending any email.

This tool accepts any hostname or IPv4 address and returns the connection status, server banner, port, security mode and the full list of SMTP features the server advertises. Read the in-depth SMTP server test guide for common connection failure patterns and a pre-send SMTP infrastructure checklist.

02 · Process

How it works

1. 1Enter the hostname or IP.Type the SMTP server hostname (e.g. smtp.yourprovider.com) or an IPv4 address. The tool accepts any publicly reachable mail server.
2. 2Open a TCP connection on port 25.The test initiates a TCP connection to the server on port 25 — the standard SMTP port for server-to-server mail transfer.
3. 3Read the SMTP banner.A reachable server responds with a 220 greeting that identifies the software and hostname. This banner confirms the server is accepting SMTP connections.
4. 4Enumerate server capabilities.The tool sends an EHLO command and captures the server's response, which lists every SMTP extension it supports — STARTTLS, AUTH methods, SIZE limits and more.

03 · Risk

Why it matters

If your SMTP server is unreachable or returning an unexpected banner, email never leaves the queue. But the problem is often invisible — your application reports a generic send failure without identifying whether the issue is a firewall rule, a misconfigured hostname, or the server being completely offline. A direct SMTP server test isolates the infrastructure layer from the application layer.

For financial services organisations, this test is part of a standard email infrastructure audit. Confirming STARTTLS is advertised verifies that your server supports opportunistic encryption for server-to-server connections — a requirement under most financial sector data protection standards. A server that does not advertise STARTTLS sends mail in cleartext between servers, which is a compliance risk.

04 · Use cases

Common ways to use this tool

• New SMTP provider verification. Before switching email service providers, run an SMTP server test against the new provider's server to confirm it is reachable and advertising expected security features.
• Delivery failure diagnosis. When a campaign shows delivery failures or the sending queue backs up, run a server test to rule out an SMTP connectivity issue before investigating DNS, authentication or content filters.
• STARTTLS compliance check. Confirm your outbound SMTP server advertises STARTTLS before a data protection or financial services compliance audit.
• Firewall and network change validation. After a firewall rule change, IP migration or datacenter move, run an SMTP server test to confirm port 25 is still accessible and the server is responding correctly.
• Hosted server monitoring. Use the SMTP server test periodically as a lightweight connectivity probe for hosted or co-located mail servers where direct access is limited.

05 · Interpretation

What to check in the result

• Connection failed. The server is either offline, port 25 is blocked by a firewall, or the hostname does not resolve. Check DNS for the hostname, confirm the server is running, and verify that port 25 is open between the test source and the server.
• STARTTLS not advertised. If the server does not list STARTTLS in its capabilities, it does not support opportunistic encryption for server-to-server connections. Configure STARTTLS on the server before the next compliance review.
• Unexpected banner hostname. The 220 banner includes the server's hostname. If it does not match the hostname you tested, the server may be misconfigured or behind a proxy altering the greeting. Align the banner hostname with the server's actual identity.
• AUTH methods exposed on port 25. AUTH LOGIN and AUTH PLAIN on port 25 should not be present on a properly configured outbound MTA. Those authentication methods belong on port 587 (submission). If they appear, review your SMTP configuration.
• SIZE limit lower than expected. The SIZE extension sets the maximum message size the server accepts. If it is lower than your typical email payload, large messages will be rejected. Increase the limit or reduce your average email size.