We currently have no plan to releasing it as an open source project.
I understand everyone wants free software, so do i, but the one behind software needs something for breakfast, or pay employees salary to continue working on it and offer excellent tech support, seems not many people care about this.
Probably no one pays or donate if you let it free
I worked on iRedMail for more than a decade, if i rely on donation, i was dead 9 years ago due to no money to buy a bread. :)
Forking and improving it is easy and doesn't take much time, maybe 1 week, 1 month or few months, but still someone needs to take care of them and offer tech support (either free or paid). Just curious, are you going to maintain it for few years? :)