If you encounter the "Problem Enabling Mass Email" error when attempting to use VanillaSoft's Mass Email integration with Vertical Response for the first time, it's likely due to an existing Vertical Response account using the same email address as your VanillaSoft account. Here's how to resolve this issue:

1. Contact Vertical Response: Reach out to Vertical Response customer support at 866-683-7842.
2. Request Conversion: Ask them to convert your Vertical Response username to be used through your VanillaSoft portal.

By following these steps, you should be able to integrate your existing Vertical Response account with VanillaSoft's Mass Email feature without any issues.