The email on your Rewind account is only used for logging in. The email draft is tied to whatever calendar you have connected.
Make sure the email app you want to use has already been installed, then follow these steps.
Open the Mail app.
Click on the Mail menu from the top toolbar, and click Settings.
Click on the General tab.
Choose an email app from the “Default email reader” pop-up menu (example: Superhuman). If you don't see your email app in the menu, choose Select from the menu, then select your email app from the file dialogue that opens.
*For directions on how to use Rewind with Gmail, click here.
For more info: https://support.apple.com/en-gb/HT201607