SMTP Test
Test SMTP connectivity to mail servers. Verify port accessibility and TLS support.
Frequently Asked Questions
What is SMTP?
SMTP (Simple Mail Transfer Protocol) is the standard protocol used to send email between mail servers and from email clients to mail servers. It operates on port 25 (server-to-server), port 587 (submission with STARTTLS, recommended for clients), and port 465 (SMTPS, legacy SSL). When you send an email, your client connects to your provider's SMTP server, which then relays the message to the recipient's mail server via SMTP.
What is the difference between port 25, 465 and 587?
Port 25 is used for server-to-server email relay (MTA to MTA) and is often blocked by ISPs to prevent spam from residential connections. Port 587 is the recommended submission port for email clients — it requires STARTTLS encryption and authentication. Port 465 was historically used for SMTPS (SMTP over SSL) and was deprecated, but has been reassigned for SMTPS use in RFC 8314. For client configuration, use port 587 with STARTTLS or port 465 with implicit TLS.
What does an SMTP banner tell you?
The SMTP banner is the greeting message sent by a mail server when a client connects. It typically shows the server hostname, software name and version, and a welcome message. For example: 220 mail.example.com ESMTP Postfix. The banner hostname should match the PTR (reverse DNS) record of the server's IP for best deliverability. An SMTP banner can also reveal information useful for diagnosis such as whether the server supports ESMTP extensions.
Why would an SMTP connection be refused?
SMTP connection refusals can have several causes: the destination port is blocked by a firewall; the mail server is offline or not listening on that port; the sending IP is blacklisted and the server is configured to reject connections from blacklisted IPs; the server only accepts connections from certain IP ranges; or the domain's MX record points to a host that is not running an SMTP server. Check the MX record, verify the server is reachable on port 25, and confirm the sending IP is not blacklisted.