Powerapps senda tölvupóst frá öðrum reikning

Office365Outlook.SendEmailV2(toEmail, subject, body, {From: fromEmail});

This parameter allows you to send Email on behalf of the others within 
your Office 365 tenant.
Doing it in this way requires Mailbox permission from the other User Account.
To give permission >> https://docs.microsoft.com/pt-pt/microsoft-365/admin/add-users/give-mailbox-permissions-to-another-user?redirectSourcePath=%252fen-us%252farticle%252fgive-mailbox-permissions-to-another-user-in-office-365-admin-help-1dbcf12f-a9de-4d1d-b0b3-a227f8a736d8&view=o365-worldwide