Recently a user had issues with Google Apps email. Emails stopped being downloaded to his machine randomly during the day. For example 2pm – 6pm emails just not  did download to Outlook via POP. Logging to Google Webmail showed the emails had actually arrived at Google so it was down to a pop error not mail transportation. I reset the POP counter by going into settings in Gmail and changing “Enable POP for mail that arrives from now on”. However this did not fix the issue. Upon further investigation with outlook and a look at the Add In’s that might be interfering with outlook , Incredimail was installed. Uninstalling this resolved the problem, so watch out installed 3rd Party Apps, I’ve also seen Smiley Central totally break the MAPI Protocol for Outlook!