Summary
This script shows how to send a delayed message with the Microsoft Graph PowerShell SDK.
The mail API in the Microsoft Graph doesn't expose any properties to defer sending a message.
Set deferred send time
Fortunately, the Graph API provides the extended properties REST API if you need to access custom data for Outlook MAPI properties that are not already exposed in the Microsoft Graph API.
Outlook MAPI provides the canonical property PidTagDeferredSendTime.
PidTagDeferredSendTime
indicates a time when a client would like to defer sending a message.