I am assuming you are using Exchange 5.5 server, since global access to mailboxes as you're referring to has been blocked in Exchange 2000 and Exchange 2003 due to privacy issues. I should caution you against opening other users' mailboxes unless you have documented procedures, management approval and employee releases (i.e., signed corporate e-mail policy) allowing you to do so. While it is commonly argued that employee mailboxes represent corporate-owned data, there are numerous legislations that vary from region to region and which govern what you can and can't do legally.
That said, there is no public Exchange Server API permitting programmatic access to the Out of Office (OOF) flag as you request, and there is no way I'm aware of to achieve this using third-party applications. I recommend you leave this to your employees, if possible, and have them at least change their voice mail remotely if they cannot report to work due to sickness.
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Both Symprex and SeliSoft have Windows applications that can set and turn off Out of Office messages. They are by no means complete solutions, nor do they allow full control like the Out of Office Assistant in Outlook 2003, but this can be done.
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