large volume of email missing

Over the last week a large volume of email is just gone, not deleted, just gone. It's over 18K emails. Opening up Mail on iPhone which is configured with IMAP settings, you saw the email count drop during sync and it went from over 22K email to just over 4.5K emails. Opening up Outlook on the PC essentially duplicated the issue. Looking at Mail on the web it shows the lower number. Where did the email go? Can it be recovered? Please advise.