You may be running out of connections to the server depending on the number
of mailboxes you are accessing. Check out
http://support.microsoft.com/kb/260141/en-us and
http://www.cdolive.com/procdo.htm

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Looking for a good book on programming Exchange, Outlook, ADSI and
SharePoint? Check out http://www.microsoft.com/MSPress/books/5517.asp
>I have a program that each 5 minutes open a session for several users and
> looks their appointments and say to another program if an user is busy or
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>
> Thanks for your help:)