This guide will cover the issue where your recipients are getting .dat files instead of the intended format in Outlook 2010 and 2013.
The cause of this is due to Outlook sending out emails in a Rich Text format.
Please follow the steps below.
Select File in your Outlook program.
In Compose Message ensure that the drop down menu next to "Compose messages in this format" is set to HTML or Plain Text.
Scroll all the way to the bottom of the screen until you locate Message Format.
Then change the drop down after "When sending messages in Rich Text format to internet recipients" to Convert to HTML format or Plain Text.
Click on OK and you should be done.