Microsoft is planning with one outlook, basically has a new experience and a new project. So what is one outlook and what is project monarch?
One outlook has the new outlook experience that centralizes everything that you might already use with outlook in terms of your native mac and windows applications so it's also called project monarch.
Microsoft traditionally have a name for their different projects and it's called monarch but there are two parts to this as they are going to power it by web and Microsoft may be going to replace windows 10 mail and calendar apps.
Outlook at the moment is basically built separately from mac and windows on devices and if all the Internet is right then, Microsoft want to basically build it from the outlook web version, so that it has the same code base across both of the different versions of it on mac and windows so they're basically building it out from the web version.
This could be for the incoming microsoft loop projects as it would provide much more relevant outlook experience and Microsoft can able to embed that and building the application, thia could also replace the mail and calendar apps that are in windows 10.
Microsoft aim to make this new one outlook the universal outlook version and it may become native os integrations inside of Outlook and Windows like offline notifications, etc.
Curently Microsoft have this in development stage and potentially could be out in march late march early april time in preview, this also means you can expect a running version in Q2 this year.