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SMTP Diagnostics
Test SMTP connection, open relay, TLS and response time
usage:
smtp-diag --server What is SMTP Diagnostics?
SMTP Diagnostics tests the SMTP connection to a mail server, verifying reachability, TLS/STARTTLS support, open relay presence, and response times. It is essential for diagnosing email delivery problems and verifying the security configuration of the mail server.
Frequently Asked Questions
What is SMTP? +
SMTP (Simple Mail Transfer Protocol) is the standard protocol for sending email between servers. It typically operates on ports 25 (server-to-server), 587 (submission with authentication), and 465 (SMTPS with implicit TLS).
What does SMTP Diagnostics verify? +
It tests the TCP connection, SMTP banner, EHLO/HELO support, STARTTLS availability, TLS version, whether the server is an open relay, and measures response times for each phase.
What is an open relay and why is it dangerous? +
An open relay is an SMTP server that accepts and forwards email from any sender to any recipient without authentication. Spammers exploit them to send spam, causing the server's IP to be blacklisted.
Why do my emails end up in spam? +
Common causes include: server IP on a blacklist, missing SPF/DKIM/DMARC, no TLS, unconfigured reverse DNS. SMTP Diagnostics helps identify server-side issues.
What does STARTTLS mean? +
STARTTLS is an extension that allows upgrading a plain SMTP connection to an encrypted TLS connection. It protects email content during transfer between servers.
Which SMTP ports should I check? +
Port 25 for receiving email between servers, 587 for sending with authentication (submission), and 465 for SMTPS. For email client use, check 587 or 465.