This is very much doable. You can do this via an automation like Mark and Kirk mentioned. I've been using this in an employee management app that I built recently and it has worked well so far.
Let's start with your checkbox:
Once the checkbox is checked, when the record is saved, the users in your "Send To" field (List-User) can receive a message that you set in your Notifications section for that table.
Let's go to the Notifications section:
You can set your notification parameters and conditions here.
Once that's done, create an automation and set your trigger definitions when a user adds or modifies a record. Set your additional criteria, if necessary. Here's you will want one of the conditions to listen for a the checkbox that is checked.
Next, set your actions and filter records based on the added/modified Record ID. Set a condition to change the checkbox value to unchecked. That should work for you.
Hope that helps.
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