There are quite a few issues of this nature reported on the internet on various forums and in all cases it turned out to be an issue with Microsoft server which in many cases suddenly started working. However, i did find some information which worked for some people. Log into Outlook.com's Settings (gear icon) > View all Outlook settings > Mail > Sync Mail and verify POP is enabled on the problem account. If it is enabled, disable it and, save the settings and go back in and turn back on.