Zerosmtp

Contents

• What's in here
• Quickstart
• Why ZeroSMTP?
• ⭐ Support ZeroSMTP
• How does this compare to other options?
• GitHub Actions
• Code Examples
• Security & Deliverability
• Star History

Arrived from the GitHub Marketplace?

This page is the README of the whole project, which is a free SMTP relay — so the action you came for is below the fold. Here it is:

yaml

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- uses: msgwing/ZeroSMTP@v1
  with:
    host: smtp.office365.com
    cert-expiry-days: '14'

It checks outbound SMTP from the runner — ports, TLS, certificate and the AUTH mechanisms offered — and fails the job when the mail server's certificate is inside the window you set. Nobody watches a mail certificate; it expires on a Sunday and the first report is somebody saying scanning stopped working.

No install and no npm: the tool has no dependencies, so the action runs the file shipped beside it. Every input, and the scheduled-canary setup that reports without failing, is in GitHub Actions further down.

What's in here

Plus what every error message actually means, and a migration guide that covers Graph API, Direct Send and paid relays — not just this one.

How big is this actually? Nobody had published a number, so we measure one every week: at least 24,960 public files on GitHub still contain a Microsoft 365 SMTP hostname. That is a count of files, not of systems that will break, and the page says exactly what it does and does not mean — along with the raw data, so you can check it rather than trust it.

Getting 535 5.7.139 Authentication unsuccessful, basic authentication is disabled? That's Microsoft switching off Basic auth for SMTP AUTH — start here. Three of the four causes are still reversible until the end of December 2026.

Want to know the day this changes? Subscribe to the dated timeline — one comment per confirmed change, nothing else.

Terminal

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npx zerosmtp-check --explain "535 5.7.139 Authentication unsuccessful"

Paste whatever your client printed — a Postfix SASL line, a Python traceback, 1102 off a Kyocera panel, or the curl: (67) Login denied that hides the server's answer entirely. They are the same refusal wearing different clothes, and it says which case you are in and whether it can still be switched back on. No install, nothing sent.

curl --url "smtps://mx.msgwing.com:465" \
  --user "$ZEROSMTP_USERNAME:$ZEROSMTP_PASSWORD" \
  --mail-from "$ZEROSMTP_FROM" --mail-rcpt "$ZEROSMTP_TO" \
  --upload-file <(printf 'Subject: Test\r\n\r\nHello from ZeroSMTP!') --ssl-reqd

That's the whole thing — no SDK, no API key, just SMTP credentials that work with anything that already speaks SMTP.

Quickstart

1. Register your free account → — 30 seconds, no credit card, no trial. You immediately get a random @msgwing.com login and password.
2. Copy .env.example to .env and fill in your credentials.
3. Run the curl snippet above (export $(grep -v '^#' .env | xargs) first), or pick your language from the Code Examples table — every example reads the same .env variables.
4. Having trouble? Check your network lets SMTP out at all — Test-NetConnection mx.msgwing.com -Port 587 on Windows, or openssl s_client -starttls smtp -connect mx.msgwing.com:587 elsewhere. That is what most first-run failures turn out to be. Then see Error messages · Troubleshooting.

Prefer not to install anything locally? Every runtime used below (Python, PHP, Node, Ruby, Go, Java, Kotlin/Gradle, .NET, Rust) is preinstalled in the included Dev Container / Codespace.

Why ZeroSMTP?

• Nothing to run and nothing to pay for. No mail server, no API key, no credit card, no per-email pricing tier to graduate into.
• Works with anything that already speaks SMTP — apps, scripts, network printers, NAS boxes, IoT hardware. If it has a "SMTP server" field, it works.
• Plain SMTP AUTH still accepted. No OAuth2 flow to implement, which is the whole point for old devices that will never get a firmware update.
• Managed reputation. Accounts are randomly generated on a domain that's actively monitored for abuse, so you're not warming up an IP yourself.
• 21 copy-paste examples across 19 languages, all reading the same environment variables, plus Ansible and Docker Compose recipes and setup guides for Windows Server, Linux, and printers by brand.
• Verifiably up — the card above is a real check against mx.msgwing.com, not a static image: every completed run is drawn, and the spacing printed on it is measured from those runs rather than copied from the schedule. GitHub delays scheduled workflows, so the cron asks for 15 minutes and the median gap is closer to 45.
• Auditable handling — where mail is processed, what is kept and for how long, and the TLS versions the relay actually accepts and refuses, measured rather than claimed: What happens to mail sent through this relay.

Good for: contact forms · password resets · CI/CD and monitoring alerts · scan-to-email · IoT and device notifications · homelabs.