smtp-probe

What is smtp-probe

smtp-probe is an email deliverability diagnostic tool for checking the technical signals that affect whether mail gets accepted or rejected. It combines DNS inspection, SMTP verification, and blacklist scanning into one utility, so you can investigate a sender domain or a specific IP without switching between separate tools. The project site describes it as an open-source, MIT-licensed, self-hosted app with no vendor lock-in, which makes it useful for operators who want full control over the workflow.

The tool can be used online in a browser, installed locally, or integrated through an HTTP API. That mix matters because smtp-probe is not just a lookup page; it is an email deliverability diagnostic tool with a CLI-first workflow and programmatic access for automation. The quick example on the site shows checks for Gmail-style DNS settings, SMTP acceptance testing, and a blacklist scan in one flow.

Key features

smtp-probe focuses on the core checks people usually need first when investigating deliverability problems:

• SPF, DKIM, and DMARC inspection for domain-level authentication signals
• SMTP address verification, including acceptance responses such as 250 2.1.5 OK
• Blacklist scanning against 29 RBL sources
• MX and PTR lookups to help confirm mail routing and reverse DNS
• Disposable email detection and free email provider detection
• Domain health checks, domain typo suggestions, and DNS record generation
• Latency probe and IP pool management features for production setups
• Multi-node deployment and role-based email detection for larger environments
• Scoring output, which summarizes results into a domain score such as 95/100 in the example

The project also includes documentation sections for Installation, Quickstart Guide, CLI Reference, HTTP API Reference, FAQ, and Deployment. That makes smtp-probe more than a single-purpose checker; it is an email deliverability diagnostic tool designed to support both quick tests and repeatable operations.

How to use it

Getting started with smtp-probe is straightforward, but the exact path depends on whether you want the browser demo, the CLI, or the API.

1. Use the online tool if you want an immediate email deliverability diagnostic tool without installing anything.
2. Install the binary if you want local or server-side checks in your own environment.
3. Run a DNS or domain check first to inspect SPF, DKIM, DMARC, MX, and PTR results.
4. Verify an address through SMTP if you need to test whether a mailbox accepts mail; the site notes that this requires port 25.
5. Scan an IP against the blacklist set when sender reputation is part of the problem.
6. Use the HTTP API or CLI reference if you want to automate checks in scripts or deployment pipelines.

The documentation also points to quickstart and deployment guides, so teams can move from a single test to a repeatable email deliverability diagnostic tool workflow. If you are handling multiple mail sources, the IP pool and multi-node features suggest the app is meant to fit production operations as well as one-off troubleshooting.

Who it is for

smtp-probe is a good fit for people who need a practical email deliverability diagnostic tool rather than a generic mail tester. Typical users include:

• Email deliverability teams checking authentication and reputation issues
• System administrators responsible for DNS and SMTP infrastructure
• Developers building mail-related automation with an HTTP API
• Operators running their own self-hosted diagnostics on Linux or macOS servers
• Teams that want to inspect blacklist status before sending from a new IP

It is especially relevant when you need to confirm whether a domain is set up correctly, whether a mailbox accepts SMTP traffic, or whether an IP has been listed on common blacklists. Because the project includes both command-line and web access, smtp-probe works for manual investigations and repeatable technical checks.

What to know before you use it

smtp-probe is powerful, but it is not a complete substitute for every deliverability workflow. SMTP verification depends on port 25, so some environments may not be able to run that check directly. Blacklist coverage is limited to the 29 RBL sources named by the project, so a clean result does not guarantee perfect inbox placement everywhere.

The tool can tell you whether DNS records, SMTP responses, and blacklist data look healthy, but it does not replace your mail provider’s logs, message trace tools, or the official admin consoles for platforms such as Google Workspace or Microsoft 365. For account-level delivery problems, authentication failures, or policy blocks, those official sources may give more precise evidence than any generic email deliverability diagnostic tool.

If you need a self-hosted email deliverability diagnostic tool that combines DNS checks, SMTP verification, and blacklist scanning in one place, smtp-probe is a strong technical option. It is especially useful when you want open-source control, CLI automation, and browser-based testing in the same workflow.