We have a user using different versions of Outlook on different computers using an IMAP connection to our mail server (which uses Dovecot).
In Outlook 2007 or other clients using IMAP, he can see all of his folders and all of his email in those folders properly. However, while he can still see the folders using Outlook 2013, he cannot see all of the email messages in those folders. For example, using Outlook 2007 he can see all 150 messages located on one folder. Using Outlook 2013 he only sees five of those messages. We can see that those messages exist on the server.
Mail to keep offline is set to all.