Emails disappearing from iPhone and iPad once outlook opened

I'm having this problem that when I open outlook on my computer all my emails disappear from iPhone and iPad. . My outlook account is pop but IMAP on iPhone and iPad. Emails delete from iPhone and iPad once outlook is opened and downloads mails. I've checked the box saying leave a copy of the message on the server as advised but this problem still recurs. I don't know what to do. I presume I can't change my outlook email to be IMAP also or is this possible? I can't seem to change the settings from POP to anything else. Can anyone help?