We've recently moved our emails to O365/Exchange Online and I wanted to see how I could enable a user to be able to send as anyone he wants thats within our domain even if the email address doesnt exist. We have a third party app where it uses SQL and VB to send emails.
I used to be able to use this powershell command to let users send as whoever they wanted:
Get-ReceiveConnector “Relay_ReceiveConnector” | Add-ADPermission -User “DOMAIN\USER” -ExtendedRights “ms-Exch-SMTP-Accept-Any-Recipient”
I'm not sure if this is possible with O365/exchange online.