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Are you having any other network issues? Have you tried using another aim account (assuming your using aim). Are you behind some sort of firewall other than the built in Mac Os X one? Also you could try resetting ichat and restarting your mac. Reset ichat by doing a spotlight search for "ichat plist" and deleting any .plist files that come up. Be aware that this will get rid of any ichat settings you had before. Then restart your mac by (this is a neat little shortcut to remember) hitting ctrl + cmd + eject. Also what version of mac os x are you using?