Why Do Emails Disappear from my Inbox?
Unread messages that are 90 days old are automatically deleted from your inbox. If you want to keep an email message, be sure to open it up so it is marked as "read." An easy way to tell whether a message has been opened is by the subject line. If the subject line of an email message is bold, that means it’s new or unread and will be deleted after 90 days. If the subject line is not bold, you have opened the message and therefore, the system will not automatically delete it.