Is it possible to drag an attachment or e-mail to an Outlook Web-Addin. The add-in is available to either Outlook 365 (Web) or the main client
My assumption is no -
System tray interception is not a viable solution, not only because of security concerns, but because of deployment reasons. Add-ins functionality and entry points should be entirely described by their manifest files and should not require the user to install any background process.
As to drag-and-drop support in general, currently drag-and-drop to the sandbox is not a part of the product. However, we track Outlook add-in feature requests on our user-voice page. Please add your request there. Feature requests on user-voice are considered when we go through our planning process.