A dynamic "mailboxing" unit and system for separating by users the sheet outputs of various users of a shared users printer (including facsimile receivers, copiers or combination units). It provides for stacking the sheets sequentially outputted from the printer in separate job sets into one or more temporarily and variably assigned "mailboxes" of a "mailboxing" job sorting accessory unit having a number of variably assignable "mailbox" bins. It may also provide a bypass for sequentially stacking unstapled user sheets directly in a mailbox without compiling and stapling. A variable display indicates the bins into which that particular user's jobs have been placed last and not yet removed. Also disclosed is automatic overflow assignments of additional temporarily designated bins for identified users, as needed, to provide effectively unlimited or "virtual bin" plural job stacking. An integral moving sheet deflector, compiler and stapler unit is shown for collecting, compiling, and optionally stapling, and ejecting job sets of sheets for separate designated users into one or more of these discrete but variably assigned "mailboxes". The "mailboxing" unit may also have "privacy doors" locking for restricting access to at least some of the mailbox bins, with electrical door unlocking of selected bins in response to entry of a user access code, and other user features.