Content Synchronization Engine is the server that will perform the IMAP sync with Gmail, Office 365, and MS Exchange. For larger deployments, it is mandatory to add remote content sync engine(s).

What is the limit specified for adding a remote content sync engine?

A. One server for every 10,000 Advanced users
B. One server for every 2,000 Advanced users
C. One server for every 5,000 Advanced users

D. One server for every 20,000 Advanced users

Reference: http://www.esna.com/services/education/IX_Messaging_10_8/TOG/RH5_IXM_TOG/IXM_TOG_System_Requirements/IXM_TOG_System_Requirements.htm