Please Note: If you schedule your send in the Outlook Web App, it will send whether you are logged into Microsoft 365 or not. If you schedule the send in the Outlook Desktop App, it will not send until you are actively logged in again.
Delay or Schedule When an Email Sends
Outlook users can delay or schedule when they want an email to send. To schedule an email to send at a future time, please click this link to the Microsoft support article:
Delay or schedule sending email messages in Outlook for Windows - Microsoft Support