In the composition window, the Options tab has a "More Options" group. One of the options is "Save copy of sent message." This is a checkbox. If you uncheck it, Outlook does not save a copy.
That's a fair number of mouse clicks, so I was thinking of writing a VBA macro to uncheck that box in one click, but I don't see a MailItem property that affects that option.
I saw the MailItem.SaveSentMessageFolder property, but it is a folder, not a boolean. I tried setting it to Nothing, but that caused Outlook to crash.
Use MailItem.DeleteAfterSubmit
property.
See all MailItem properties and methods at https://learn.microsoft.com/en-us/office/vba/api/outlook.mailitem
You can see live property values of the various Outlook objects using OutlookSpy (I am its author) - click the Item button.