Compose an email and schedule it to be sent out automatically at a certain date/time.
Hi @solution69 - are you suggesting this feature for Outlook or for PowerApps? As jwells also mentioned, looking at Microsoft Flow can do this type of automation as well - you can see the templates available for Office 365 here - https://us.flow.microsoft.com/en-us/connectors/shared_office365/office-365-outlook/
Are you looking for the user of the app to be able to set a specific time to send a custom message? Or a specific message to be sent out based on some other criteria such as current time, etc?
Take a look at Microsoft Flow if you can. You've got 'Delay Until' actions, along with other Date/Time actions and whatever email actions you'd need to send messages and such, all of which can be fed data from your PowerApps app. If you need a hand with it, toss me some details and I can try to help out.