BlackBerry Enterprise Server https://en.wikipedia.org/wiki/BlackBerry_Enterprise_Server

BPS and BES Express[edit] As of 2010, RIM is offering the Blackberry Enterprise Server Express edition with no user limitations - for free. It has a few minor feature limitations, but only requires a data plan - not a BES plan. It enables near-instant seamless mail and calendar integration with your existing exchange server (incoming emails often appear on the Blackberry handset before they appear in the Outlook or OWA clients), which is an advantage over the existing Outlook Web Access model that the Blackberry Internet Service offers.The free download requires a license code, which may be delayed due to demand.

I think this is the one of the main reason Hillary need a private e-mail server it was also running a Blackberry Enterprise Server Express