If you send an email with WebDAV using the Exchange Mail Submission URI (see here for an example), the mail is placed in the Sent Items folder after the mail was sent. To prevent this, you can add a SAVEINSENT header to the MOVE request that is used to move the message from the drafts folder to the mail submission url. If you set value of the header to F, the mail will not be placed in the Sent items folder.
There is another header which controls the behavior: The SAVEDESTINATION header. You can set this header to the address of another folder in the same mailbox. The mail will be saved in that folder instead of the default Sent Items folder.