UPDATE: This issue has been fixed in the latest MailMax/WEB updates.
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Because of the way IE caches its history, when a user hits the built in browser back button, the message delete is called again, which deletes unread messages.
There are two work arounds:
1. Don't use the browser "back" button, only use the controls within MailMax/WEB. After reading a message, click the goto folder button, or the check mail button.
2. On the machine running MailMax/WEB, go to HKey_Local_Machine/Software/SmartMax Software/MailMaxweb, change the key labelled "SingleFrame" from a 1 to a 0. This resolution opens new email messages in a separate window while leaving the message list viewable and avaible.