How do I properly set up IMAP email on Windows Phone?

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So my dad and I recently bought a Lumia 640 and so far we're really happy with it. I had no problems setting up my Outlook.com email and it all went automatically. My dad however has a different email account and therefore has to use IMAP. We used the IMAP settings provided by the email provider and it worked.. sort of.

The problem is that he does get new email, but his old mail doesn't sync and this while he has the exact same settings as me (download email from the last 7 days). Also, when he has read the new email it also disappears. He can only see the latest email, not even all the other unread email. I tried to mess around with the settings but to no avail. Has anyone had this problem before? Is there a workaround?

 

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Fixed this problem:
Apparently my dad had configured his email also on Outlook on his desktop but used POP instead of IMAP. His POP settings were that there were no copies of his emails left on the server after downloading in Outlook.. So there won't be any email to download for the Lumia. A way to fix this is to change the POP settings in desktop Outlook to leave a copy of the emails on the server or to change the desktop Outlook to also use IMAP.