Outlook has a feature which can attempt to detect if you have forgotten to attach that very important document to your email. Whether you like it or not, here is how to both enable as well as disable it.
How to Enable or Disable The Forgotten Attachment Reminder in Outlook 2013
Open Outlook and click on File to enter the backstage view.
Here you will want to click on options.
When the Outlook options dialog opens, switch over to the mail section on the left hand side.
Then scroll down to the “Send messages” section and either tick the last check box to enable the warning, or untick it to disable the warning message.
That’s all there is to it.