There is a way to do this, however the platform does not have a feature specifically built to send an email to all clients at once. We've chosen not to build this in large part because this would create a significant risk of accidental HIPAA breach or accidental use.
Note: For more information about all of the ways we work hard to keep your clients' data safe, please see this guide: SimplePractice Trust & Security.
There are some customers who are sending bulk emails to clients outside of SimplePractice. Using the Data Export tool, you have the option to export client contact information in bulk in the vCard format (.vcf). You can find step-by-step instructions on how to do this in this walkthrough: Data export: Exporting client information. You can then arrange the data as you need into a spreadsheet and use a mail merge function. If you choose to do this, please be mindful of where and how you share information via email with clients.