[Answered ]-Create .msg file with task without having outlook installed

Well the MSG file format is defined here: https://msdn.microsoft.com/en-us/library/cc463912(v=exchg.80).aspx but like all microsoft documention it’s anything but readable. All you need to do is to write a bit of python code to produce a file that fits the given format.

Why not create an EML file? It is MIME, so there are hundreds of libraries out there. Outlook will be able to open an EML file just fine.

In your particular case, create a MIME file with the vTodo MIME part as the primary MIME part.