Open Outlook
Go to the Sent Items folder
Open the message you want to unsend
Select File
Select Info
Select Resend or Recall
Select Recall This Message
Choose Delete unread copies of this message
Optionally select Tell me if recall succeeds or fails for each recipient
Select OK
If prompted, confirm the recall
Note that recall only works in some Microsoft Exchange or Microsoft 365 accounts
Note that recall usually only works if the recipient has not opened the message