Have you recently set up a new email account or mail collection service. If the account was set up as a standard POP3 account then what happens is that the new account will download all mail messages from the Inbox and then delete those messages from the online server. This means that any device connecting to the mailbox will see no older emails in the Inbox although, if connecting as an IMAP email account will see other standard and personal folders that are in the online mailbox.
