Microsoft has finally added the "Undo Send Feature" to Exchange Online. Gmail has had this feature (officially) for a couple of years already. The "Undo Send Feature" lets end-user add a custom "grace" period (5 to 30seconds) and the mail, as shown here:
During this period, the message stays in the Drafts folder and can be recalled by clicking the Undo button:
Basically when the delay period elapses (when the progress bar is full) OWA sends the message and it's transmitted to the server.
As a sidenote, please note the warning message:
"If you close your browser or put your computer to sleep during the period that you've chosen for cancelling your message, the message won't be sent"
This leads me to the conclusion that it's all is being handled in your browser (and that 10 seconds is enough of delay).
Please not that as of today it's not possible to customize this by Powershell ;(.