I have the same issue, but what worries me: I cannot sync my Outlook.com account. But why can't I sync my personal mail account which not on Outlook.com, not even from Microsoft? Do all These mail syncs go via MS Servers?
edit: forgot to mention, my Modern Mail app on Windows 8.1 syncs my non-MS Account propperly, just my Windows Phone does not!?
edit2: sorry for this, just found the issue: had my Outlook.com mail account and my personal non-MS one connected on my phone. After disconnecting them and a reboot my non-MS account syncs again while the outlook.com account still does not.