The disclosed embodiments provide for an email and messenging system that allows a user composing a message to select an application which will appear next to the composition screen at both the sending and receiving computer. The application is specified as an attachment at the sending client and when received at a receiving client of a user with an enhanced or compliant mail/messenging account the application will be executed at the receiving client, thus customizing the functionality of the receiving client computer and the mail experience between sender and receiver. An unapproved application sent as an attachment will not be executed and/or alter the receiving client interface. Similarly, the application will simply appear as a standard attachment at a non enhanced recipient client/account.