By default Outlook 2016 now attaches files as links to cloud storage wherever possible. So if you attach files from OneDrive it will try and send the file via OneDrive, at present there is no way to disable this altogether.
Whilst it can't be stopped altogether at present there are a couple of workarounds that will prevent it.
Below are the workarounds;
First possible workaround is just to click the drop down on attached files and then select "Send as copy" each time a file is attached.
This is outlined with pictures here: https://community.office365.com/en-us/f/172/p/412847/1025077#1025077
Alternative workaround - we can add the "classic" attach button to your Outlook. This won't replace the new one, so you'll have to remember to use it instead, but it will do away with the need to change each file to "Send as copy". http://www.msoutlook.info/question/classic-attach-file-button