FAQ

Does smtp-probe work without port 25?

Yes. The check command (DNS-based diagnostics) works everywhere — no special network requirements. The verify, probe, and blacklist commands require outbound TCP port 25 access, which is blocked by most cloud providers.

Why does SMTP verification return 5xx even for valid emails?

Some mail providers return 5xx as a catch-all rejection to prevent address harvesting. This does not always mean the mailbox doesn’t exist. Try with the --pool flag to avoid being rate-limited by a single IP.

How do I get port 25 access on cloud servers?

Most providers require you to submit a support ticket or form:

• AWS: Request via Support Center → Create case → Service: EC2 → Port 25 access
• GCP: Fill out the port 25 request form
• Azure: Open a support ticket
• Alibaba Cloud: Submit a port 25 unblock application

What’s the difference between check and report?

• check — DNS records only (SPF/DKIM/DMARC/MX/PTR)
• report — DNS records + blacklist scan for all MX IPs

Can I run smtp-probe as a long-running service?

Yes. Use the serve command to start an HTTP server running in the background. See Installation for setup instructions.

How do I keep disposable email lists up to date?

Run the auto-update cron installer:

smtp-probe disposable update --install-cron

This adds a daily crontab job that re-downloads lists older than 7 days.

Does smtp-probe store any data?

Pool configuration is persisted at ~/.smtp-probe/pool.json. No other data is stored.

Is smtp-probe open source?

Yes. smtp-probe is MIT licensed. Source code: github.com/monto-fe/smtp-probe