The error message "Something has gone wrong. We were unable to connect to your custom mailbox. See details" typically indicates a problem with the connection between your email client and your custom mailbox. Here are some steps you can take to troubleshoot the issue:

1. Check your internet connection: zombie  Ensure that your internet connection is stable and functioning properly. A poor or unstable connection can cause intermittent connection issues.

2. Verify your mailbox settings: Double-check that the settings for your custom mailbox are correct. Make sure that you have entered the correct username, password, server settings, and port numbers for your email account.

3. Clear your cache and cookies: Clearing your browser's cache and cookies can help to fix any temporary issues with your browser. Try clearing your cache and cookies and then try connecting to your custom mailbox again.

4. Try accessing your mailbox from a different device: If you are still experiencing issues, try accessing your mailbox from a different device or browser. This will help you determine if the problem is with your device or browser.

5. Contact your email service provider: If none of the above solutions work, contact your email service provider or IT department to determine if there are any known issues or outages with their service. They may be able to provide additional troubleshooting steps or support.

If you continue to have issues, you may want to seek the assistance of a technical expert who can help you diagnose and fix the problem.