There is no set limit for mailboxes, storage space is the limiting factor. A 512MB flash allows around 80 hours,storage on a PS1 should not be an issue. The limits are for mailboxes with UM.
Although the new BVM and Nupoint can reside on the same system Mitel recomends using the Nupoint for VM storage and the BVM for STARS, CRAs Queue position and wait time only.
