A: There are several reasons for this:
A user using POP3 client software like Outlook can set it to read and download email but still store a copy of the mail on the server to be accessed in the future.
Users use our webmail feature only, but they do not delete any emails or download it.
Some web mail providers, like Hotmail, allow users to grab mail from our servers and still keep a copy on the server. Therefore, the mail server still has a copy of mail that has been read/downloaded.