SMTP Test
What Is an SMTP Test?
An SMTP (Simple Mail Transfer Protocol) test connects to a domain's mail server and performs the initial stages of the email delivery handshake. This verifies that the mail server is reachable, responsive, and properly configured to accept email. Our tool looks up the domain's MX records, connects to the highest-priority mail server, and tests the SMTP conversation without sending any actual email.
The test begins by establishing a TCP connection to the mail server, typically on port 25. The server responds with a banner message that usually identifies the mail server software and hostname. Our tool then sends an EHLO command, which is the extended greeting used in modern SMTP. The server responds with a list of supported extensions, revealing its capabilities and configuration.
One of the most important extensions to check is STARTTLS, which enables encryption for the SMTP connection. When STARTTLS is supported, the connection is upgraded from plain text to TLS before any email content is transmitted. This protects email in transit from eavesdropping. Most major email providers now require STARTTLS support, and some will refuse to deliver email to servers that do not support it. Our tool reports whether STARTTLS is available and which TLS version the server supports.
Response time is another key metric provided by the SMTP test. Slow mail server response times can cause sending servers to time out and fail to deliver email. The SMTP protocol has specific timeout requirements - for example, a server must respond to the initial connection within 5 minutes. Our tool measures the actual response time so you can identify performance issues before they impact delivery.
SMTP testing is an essential part of diagnosing email delivery problems. If email to your domain is bouncing or being delayed, an SMTP test can reveal whether the issue is at the network level (server unreachable), the protocol level (misconfigured SMTP), or the security level (missing STARTTLS). Combined with our MX Lookup and SPF Checker, you can diagnose the full chain of email delivery.
Frequently Asked Questions
What is an SMTP test?
An SMTP test connects to a mail server and performs the initial SMTP handshake. It sends an EHLO command, checks for STARTTLS encryption support, and measures response time. This helps diagnose email delivery issues and verify mail server configuration.
What is STARTTLS?
STARTTLS is an SMTP extension that upgrades a plain-text connection to encrypted TLS. When supported, the connection is encrypted before email data is transmitted. Most modern mail servers support STARTTLS, and many providers require it.
What does the EHLO response tell me?
The EHLO response lists supported SMTP extensions: STARTTLS (encryption), SIZE (max message size), 8BITMIME (character support), PIPELINING (command batching), and AUTH (authentication). This helps diagnose compatibility issues.
Why is my SMTP connection failing?
Common causes: server is down, port 25 is blocked by ISP or firewall, MX records are misconfigured, DNS resolution is failing, or the server is rejecting connections from your IP range.