In System Notes, value of Custom Check Box field is either 'T' or 'F' only.
To send an email alert when a Custom Check Box is set from 'false' to 'true', follow the steps below:
1. Navigate to Lists > Search > Saved Searches > New
2. Click Customer
3. In the Email tab, mark Send Email Alerts When Records are Created/Updated
4. In the Specific Recipients tab, select the recipients
5. In the Updated Fields tab, specify the following:
Field = Custom Check Box field
When Old Value is... = (blank) or F
When New Value is... = T
6. Click Save
Note: This is also applicable on other types of saved searches.