Welcome to the Community. I'd be glad to lend a hand with sending an email to all of your customers.
To ensure that I'm on the same page, are you wishing to send a new email to all of your customers, such as advertising a promotion? If so, you can accomplish this with the following steps:
I've included a brief video as an example:
In my experience, you will need to have Outlook installed and set as the default email service on your computer.
Should you need to batch send sales transactions, please refer to the following link: How to Email or Print Multiple Invoices and Sales Forms at One Time.
I hope that this response finds you with a smile. Let me know if I can be of additional help.