Use Public or shared computer if you are not using your own computer to connect to OWA because it offers some additional protection. If there is no activity between your computer and the Exchange server for 30 minutes — such as sending or receiving e-mail — OWA will require you to log in again. After logging in, you will be able to continue from where you were.
If you are using your own computer, the Private option is more convenient. It offers a much longer time frame before OWA would require you to log in again.
Weigh the convenience against protecting your online security. Having to log in more often helps reduce the likelihood that someone else can access your e-mail, calendar, and everything else in your Exchange account. Imagine the problems, for example, if someone used your Exchange account to send an offensive or threatening e-mail in your name.
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