Single direction synchronisation to/from desktop

I’m new to nexcloud and installed a test repository.

What I wanted to achieve is a mailbox system for users, that works as followes:

2 … n users and each user has it’s own directory which is used as a mailbox.
Every authorized other user can post a file into the mailbox and the file is automatically downloaded to the user’s desktop.
If there are only two users, this is easy manageable, but if there are three or more users this will be unefficient, because everything is always syncronized.
A solution would be to allow only single direction synchronisation to other users then one self.

Does anybody have a solution for this?
Or is this a suggestion for the desktop and mobile app?

Greeting from a newby