The first thing I would check is DNS. Make sure DNS on the client is set properly. Then, check your IE proxy settings. Specifically, if you are using a proxy server, you need to do two things: 1. Bypass the proxy for local addresses. 2. Add all internal subnets to the IE Proxy Exception list (use the Advanced button under Proxy settings to do this). Then, try to log onto IM again. Also, you didn't say what clients these are, but if they are Windows 2000 or Windows XP, also try doing: ipconfig /flushdns ipconfig /registerdns and see if that helps.
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