When you send an email, Outlook sends the original email to the recipient and saves a copy of that email to the "Sent" folder. This default behavior of course makes sense and will probably never change, but that does not mean that if you do not like it you can not change it.
Outlook lets you turn it off mode so that it does not save copies of your sent emails in the Sent folder. If you usually delete what is in the Sent folder, then you are covered and do not need to do the procedure below.
We are interested in setting the process as the default for all emails, but you can also do it for a single email. In Outlook, open the "Options" tab and click "Save Sent Item To", which will give you the option to specify a specific folder or not save the sent email at all.
Of course we can not think of many conditions that will suit you you are doing this. However, if you still want to do this for all emails, see how.
How Outlook stops storing your sent emails
To prevent Microsoft Outlook from saving a copy of your sent emails, go to File> Options.
On the "Mail" tab, scroll down to the "Save Messages" section and uncheck "Save copies of messages in the Sent Items folder."
Now click on "OK" at the bottom right of the "Options" window.
Source of information: howtogeek.com