SMTP Tester
Step-by-step SMTP diagnosis — connection, TLS, auth and live test delivery.
Credentials are used once over HTTPS for the live test — never logged or stored. Rate limited to 5 tests/minute.
Debug email delivery the way mail admins do — by watching the actual SMTP conversation. Each step shows the real server response and timing, so "email not sending" turns into a specific, fixable answer. Once your sending works, verify your DNS posture with the SPF / DKIM / DMARC Checker so the mail you send actually lands in inboxes.
Frequently Asked Questions
What does the SMTP tester check?
It walks through a real SMTP conversation step by step — TCP connect, EHLO handshake, STARTTLS encryption, authentication with your credentials, and optionally sending a real test message — showing exactly which step fails and what the server said. That pinpoints whether the problem is DNS, firewall, TLS, credentials, or sending limits.
Which port and security setting should I use?
Port 587 with STARTTLS is the modern standard for authenticated sending. Port 465 uses implicit SSL/TLS. Port 25 is for server-to-server mail and is blocked by most ISPs and cloud providers. The tool auto-fills the right port when you pick a security mode.
Is it safe to enter my SMTP password here?
Credentials travel over HTTPS to our API, are used once for the live test, and are never logged or stored. That said, best practice for any third-party tester: use an app-specific password (Gmail and most providers support them) and rotate it after testing if you are cautious.
Why does authentication fail with my correct Gmail/Outlook password?
Major providers block plain passwords on SMTP. Gmail requires an App Password (with 2FA enabled); Microsoft 365 requires SMTP AUTH to be enabled per-mailbox plus an app password. Check those settings before assuming the credentials are wrong.
Why can I not send a test without username and password?
Unauthenticated sending through arbitrary servers is exactly how spam relays work, so the tool refuses it by design. Test sends only go through servers that accept YOUR credentials — meaning you can only send via infrastructure you control.