The Carnegie Mellon University Cyrus project has produced a full featured, large system (1000 - 100000 mailbox) oriented server. This server is complicated. The compensating factor is that it has some very sophisticated administrative utilities. The disk structure that it uses is the most amenable to larger numbers of virtual servers.
Previous experience with this system suggests that it may have problems with 4.4BSD based system that have 64 bit file offsets. The most recent Cyrus release may solve this problem.
Base installation and testing of this software will take one day. Modifications to accomplish the virtual servers would take an additional three days (estimate).