exchange server 5.5 stops responding [closed]

What could be the reason for stop working?

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We have not installed any updates or software since last more than 2 years.

Asked and Answered: You have neglected your environment for at least 2 years - You should expect it to be falling apart.
Server environments DO NOT continue to just run forever, despite what we admins like to say in our smoke- and whisky-filled poker games: They require love and attention to ensure that they keep operating.

Running Exchange 5.5 (which I know is past its support life) on Win2k3 (which I'm also pretty sure is past its support life - SP2 may have some time left on the death clock) is a recipe for disaster. The best advice anyone can give you is "Start planning your migration off this obsolete technology. NOW."

If you really have your heart set on troubleshooting this mess I would suggest checking your system logs for information about the problem. What you have described ("losing connection with other outlook clients") is so generic that I can't give you any better guidance, but if you find something relevant in your logs it might point to something that people can help you with.
Bear in mind however that Exchange 5.5 is so ancient (1997!) that most of the knowledge pool for it has been drained as people moved on to newer, better releases of Exchange. There simply may not be anyone around who remembers enough about this software to help you.