On 3/27, Andrew Salamatov published Customizing the compose experience with mail apps for Outlook. In his post, he talks about how you can customize your compose experience in apps for Outlook.

Compose apps are available for all our customers on Office 365 and for on-premises customers that have installed the Service Pack 1 for Office 2013 and Exchange 2013.

Check Andrew’s post in the Exchange dev blog and build great apps for Outlook!