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Discussing in eXo Chat by Sending and recieving instant messages induces to register the chat messages history which could be easily accessible later through the chat room.
To visualize old chat messages, all you need to do is to scroll up the chat window, a loading icon appears the time to load up to 200 old message. When the loading icon disapears this means that the 200 messages was loaded and you can read them.
The number of loaded chat messages is set by default to 200, it could be configurable in exo.properties.