Have you ever deleted any email from outbox. There are several reasons. Check for all office updates. Try sending a mail to your own email id and observe the same thing is happened!
Another way you may create a new outlook profile and setup email account & again check the whole thing.
If all wll not work, try for the below….
In Mail > Tools menu > click Rules and Alerts > E-mail Rules tab, under Rule, uncheck or clear the check box where “Cc the message to people or distribution list”.

For better info, please link here.