Yes, absolutely. First, you might want to draft the message by creating a new template under Messages > Templates.
Next, go to Clients > List, click 'Select all', then 'Email', select the template you created, and send it out. The message will be sent in a "bcc" format so that your clients will not see other clients' names and email addresses.
To safeguard against spam, Full Slate limits the number of recipients on a bulk email to 500. If your opt-in list is larger than that, you can still send to the entire list by breaking it down into smaller batches. To select a range of records, click one at the top then hold down “shift” and click another at the bottom; all the records in between will be selected.
Also, you might want to ask your clients if they want to receive promotional emails from you. This sounds pretty basic, but it helps ensure your messages are welcome. And it’s easy to do – you can add an “Opt-in” checkbox to your online scheduling form under Setup > Booking Details (just don’t make it required). Or you can ask for permission in person at the end of an appointment and check the box yourself under Clients > Details. Then before you send a mass email from Clients > List or export your mailing list, filter to “Opt-in only.”