Ever accidentally delete your in-box? (Happens to me all the time.) You can prevent this disaster, or facilitate a switch of e-mail clients, by adjusting a setting in your e-mail program. Most programs default to deleting e-mail from the ISP's mailbox as you download it, but they can be reset to keep mail on the server for a set number of days. It's a great way to check mail from more than one system-such as home and office computers and a Web phone-without having to delete junk mail more than once.