The best replacement for enterprise server is blackberry internet service.

The best alternatives to enterprise server are rated as following:
★★★★★  blackberry internet service
★★★★  activesync
★★★★  desktop redirector
★★★  standard server
★★★  opensuse
★★★  blackberry desktop redirector
★★  internet service
★  blackberry internet server
★  redirector

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Speaking about enterprise server, one probably shouldn't forget about multiple exchange servers.

Other search terms related to enterprise server and their ratings are:
★★★★★ multiple exchange servers
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enterprise server
blackberry internet service
activesync