The error message you received indicates that the cloud provider (such as Google or Amazon) has blocked outgoing traffic on port 25, which is the default port used by Postfix (a popular mail transfer agent) to send emails.
Cloud providers often block port 25 as a security measure to prevent their infrastructure from being used for sending spam or malicious emails. However, this restriction can cause issues for legitimate email servers like Postfix running on their platforms.
To resolve this issue, you typically have two options:
Contact the cloud provider: As the error message suggests, you can contact the cloud provider's support and request them to open or unblock port 25 for your server or instance. They may have a process to verify that you intend to use the port for legitimate email services and then grant you an exception to allow outgoing traffic on port 25.
Use an alternative submission port: Many cloud providers recommend using an alternative submission port instead of port 25 for sending outbound emails. Common alternatives include ports 587 (submission) and 465 (SMTPS – secure SMTP over SSL/TLS). You would need to configure Postfix to use one of these alternative ports and potentially set up authentication if required by the provider.
Here are some general steps you can follow:
- Check your cloud provider's documentation or contact their support to understand their recommended approach for sending outbound emails from their platform.
- If they allow unblocking port 25, follow their process to request an exception.
- If they recommend using an alternative submission port, update your Postfix configuration to use the recommended port (e.g., 587 or 465) and any necessary authentication settings.
- Restart the Postfix service after making the configuration changes.
By following the cloud provider's recommended approach, you should be able to resolve the issue and send emails successfully using Postfix on their platform.