Free tools
Open relay checker
Enter SMTP hostname and port (usually 25). Only test servers you own or have permission to test. This is a diagnostic UI preview.
What this checks
Simulated relay attempt without authentication, banner response, and whether the server accepts external RCPT TO.
Why it matters
Open relays allow anyone to send through your server. They destroy IP reputation and often lead to immediate blacklist listings.
Results
Common issues
- Misconfigured Postfix or Exim allowing relay from local networks too broadly.
- Compromised server used as spam gateway before detection.
- Confusing open relay with legitimate authenticated submission on port 587.
How to fix
- Require AUTH for submission ports and restrict relay to trusted IPs only.
- Patch and harden mail server software; monitor logs for abuse.
- Request delisting after closing the relay and fixing the root cause.
How MailFleet helps
MailFleet sends through configured provider profiles — not open relays — and logs SMTP errors when providers reject mail.
Related free tools
Delivery note: MailFleet helps users manage campaigns through their own sending providers. Delivery outcomes depend on sender reputation, DNS authentication, content quality, recipient engagement, list quality, provider rules, and mailbox filtering systems.
For regulatory context on commercial email in the United States, see the FTC CAN-SPAM compliance guide.
Frequently asked questions
An open relay accepts and forwards mail from unauthorized clients to external domains. It should reject unauthenticated relay attempts.
Get started
Prepare and control campaigns with MailFleet
Run integrated diagnostics, manage providers, and monitor campaigns from a professional desktop app.
Available on
- Windows 10+
- macOS 12+
- Linux (DEB & RPM)
Same desktop experience across every platform — one workflow for your entire team.