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How to Troubleshoot SMTP Connection Issues | Postmark Support Center

How to Troubleshoot SMTP Connection Issues

🛠️ Troubleshooting SMTP Connection Issues with Postmark

If you're running into trouble connecting to Postmark via SMTP, this guide covers a few key steps to help identify and resolve the issue.

We’ll walk through some common fixes and share what to look out for on your end. If things are still unclear after trying the steps below, feel free to reach out—we’re here to help.


🔗 Helpful Resources


✅ SMTP Troubleshooting Checklist

If you're seeing connection issues, here are a few things to double-check:

1. Try alternative SMTP ports
Postmark supports the following ports for SMTP: 25, 2525, and 587.
If one isn't working, try switching to another to rule out port-specific restrictions.

2. Identify your SMTP client
Different SMTP clients behave a bit differently. Knowing which one you’re using (e.g. Outlook, Thunderbird, a library like Nodemailer, etc.) helps us offer more targeted support.

3. Check your TLS version
Postmark requires TLS v1.1 or higher. If your setup is using TLS v1.0 or is hardcoded to a specific version, that can cause connection failures—even if the server supports newer versions.

4. Allowlist Postmark’s SMTP IPs
Firewalls or outbound filters can block SMTP traffic. Make sure your network is set to allow connections to Postmark’s SMTP IPs.


🧪 Test with an Online SMTP Tool

To confirm whether the issue is with your SMTP client or the connection itself, try sending a test message via smtper.net.

  • Be sure to check "Use Secured Connection" and "Use Authentication"

  • Here’s what that looks like: Screenshot

If the test message goes through successfully, that’s a good sign that Postmark is accepting connections and the issue may be related to how your SMTP client is set up.


📸 Still Having Trouble?

If SMTP is still failing after these steps:

  • Take a screenshot of your SMTP configuration page

  • Note any specific error messages you’re seeing

  • Share those details with us—we’ll be happy to take a closer look

Getting logs or screenshots is often the fastest path to a solution.

Last updated August 14th, 2025

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