Appengine mail couldn't connect to host port

A frustrating error!

The "couldn't connect to host port" error when using App Engine's Mail API is usually caused by a network connectivity issue or a misconfiguration. Here are some troubleshooting steps to help you resolve the issue:

1. Check your App Engine project settings:
   • Ensure that the Mail API is enabled for your project. Go to the App Engine dashboard, click on "Settings" (gear icon), and then click on "APIs & Services" > "Mail API". Make sure the toggle is set to "On".
   • Verify that your project has a valid billing account. The Mail API requires a billing account to function.
2. Check your network connectivity:
   • Ensure that your development machine has a stable internet connection.
   • If you're behind a proxy or firewall, check if there are any restrictions on outgoing connections to Google's servers (e.g., smtp.gmail.com or smtp.googlemail.com).
3. Check your Mail API configuration:
   • Verify that your Mail API configuration is correct. You can check the configuration by running the following command in your terminal:

     gcloud app config get-value mail/smtp-relay

   • Make sure that the smtp-relay value is set to smtp.gmail.com or smtp.googlemail.com, depending on your project's configuration.
4. Check your App Engine instance's network settings:
   • Ensure that your App Engine instance has the necessary network settings to connect to the Mail API. You can check the network settings by running the following command in your terminal:

     gcloud app instances describe <instance-name> --format="get(networkSettings)"

   • Verify that the networkSettings value includes the necessary settings for outgoing connections to the Mail API.
5. Try sending a test email:
   • Use the gcloud app mail send command to send a test email to verify if the issue is specific to your code or a general connectivity issue:

     gcloud app mail send --from=<your-email-address> --to=<recipient-email-address> --subject="Test email"

   • If the test email fails, it may indicate a connectivity issue or a problem with your Mail API configuration.
6. Check the App Engine logs:
   • Check the App Engine logs to see if there are any errors related to the Mail API. You can do this by running the following command in your terminal:

     gcloud app logs read --format="json" --filter="Mail API"

   • Look for any errors or warnings related to the Mail API that may indicate the cause of the issue.

If none of these troubleshooting steps resolve the issue, you may want to reach out to the App Engine support team or the Google Cloud Support team for further assistance.