Open Outlook and go to your Sent Items folder
Double-click 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, choose whether to replace the message with a new one
Note that recall only works in some Microsoft Exchange or Microsoft 365 accounts
Note that recall does not work if the recipient has already opened the message or uses incompatible email systems