Hi, 

I would like to send a mail to the assignee of an activity of type mail.

Use case: For fleet services, I've set activities to plan maintenance for each vehicle. This was scheduled for the whole year. Now I would like to receive a mail X days before the due date of the activity in order to start the planning.

Using odoo 17, I've tried it with automation rules but I got the following error: Validation Error This action can only be done on a mail thread models. Is there anyone who has experience with this? 

Thanks!