There are differences in the way Outlook renders HTML from version to version. In Outlook 2013, any full-width template pieces used within our template builder appear to blow out beyond our standard email template width. This is because Outlook 2013 overrides a fixed container width with the image width inside that container. To combat this, we have implemented MSO conditionals to declare the width of any full-width image, rather than declaring 100%, so that it fits our standard email template width on desktop. Because of this, however, any email created in Outlook 2013 that uses a full-width image template piece will not be responsive on mobile.
Note that because Outlook reads MSO conditionals based on the version that the email template is created in, any template using a full-width image created in versions greater than Outlook 2013 will appear to be blown out when received in Outlook 2013.
To avoid this rendering discrepancy, it is recommended that all senders and receivers are using the latest desktop version of Outlook.