More accurately, anyone is free to fork modest. The source code may be available under an open licence, but it's not a community project (ie, development is driven cathedral-style based on internal design specifications that we can't even see). Moreover, Nokia seems to has decided that it's adequate in its current state and has moved most of the developers to other projects.

There seems little point in spending all the time and effort required to get to grips with the code (and tinymail, camel etc) only to be told that your patch goes against the design or (to get back on topic) that things will be done differently in Fremantle and your patch doesn't apply.