IMAP4 Messages being deleted 'mysteriously'

> I have an issue with just one user. Intermittently, a whole bunch of emails are just deleted from his inbox. Is there any way to tell who deleted them?

If they were deleted recently, you can undelete the messages from by editing the user, going to the Advanced tab and using the 'Undelete' options there.

However, if you want more information, before undeleting the messages you could go to Settings -> Database -> Query and type the following query (copy & paste is probably better):

select msgid, deletedtime,deletedreason from messages.foldermessages m inner join messages.folders f 
  using(folderid) inner join users.users u on u.usernumber=f.userid where 
  username='<username>' and folder='Inbox' and deletedtime is not null order by deletedtime desc;

(change '<username>' to the VPOP3 username).

This will display when the message was deleted, and 'why' (usually something like 'IMAP--'). That may show if it was a different user (eg for shared folders), or if it was deleted from a different computer/device or Webmail, or a POP3 client or something.

In VPOP3 v7 and later, you can do a similar thing in Settings -> Diagnostics -> Message Search. This will let you search the entire message store, including messages in the VPOP3 recycle bin.