In Outlook, you can specify the folder where sent email items are saved.
Outlook 2016 and Outlook 2013 for Windows uses Exchange ActiveSync (EAS) to connect to Outlook.com (formerly Hotmail), which doesn’t save sent messages in the folder you sent them from. Instead, Outlook stores them in your Sent Items folder. You cannot change where sent email messages are saved when using an IMAP account, although you can turn off saving of sent messages entirely.
If you have a mail account that is not an Outlook.com account or another Exchange ActiveSync account, you can use the steps below to save your sent messages in the originating folder.
Change where sent email messages are saved when using an Exchange account
Click File > Options > Mail.
Under Save messages, make sure the Save copies of messages in the Sent Items folder check box is selected.
Select the When replying to a message that is not in the Inbox, save the reply in the same folder check box.
Turn off saving sent items in an IMAP email account
If you are using an IMAP account with Outlook, you may notice that sent messages are being saved twice. To resolve this, you can turn off saving of sent items in Outlook.
On the File menu, click Account Settings > Account Settings.
Select the IMAP, and then click Change.
Click More Settings.
In the Internet Email Settings dialog box, click the Advanced tab, and then select the Do not save copies of sent items.