Microsoft is working on a lightweight e-mail / chat hybrid app called Flow. The app, first spotted on a public download site despite being marked confidential, promises fast, fluid, natural conversations without subject lines, salutations or signatures.

In a nutshell, Flow will allow users to communicate with people on their e-mail contact list in an informal way, much like text messaging. Correspondence will also be saved in Outlook so you can pick up a conversation you first started on your mobile device right on your desktop or notebook.

Only those conversations started in Flow and their replies will show up in the app; all other correspondence from your inbox will remain there. All that’s needed to get started is an e-mail address. If the recipient of your Flow message doesn’t yet have the app, the message will simply arrive in their inbox as a traditional e-mail. Think of it as informal e-mail for the modern era.

Given the tremendous success that standalone messaging apps have found over the past few years, it’s no surprise to see Microsoft trying its hand at what’s essentially the same thing.

Twitter user @h0x0d was the first to find the app which will apparently be an iOS exclusive, at least initially.

As ZDNet points out, this isn’t Microsoft’s first attempt at modifying existing platforms into something a bit more lightweight. Last October, the company launched Skype Qik, a streamlined version of Skype that offers a simpler interface with brief chats in mind.