We had this happen a few months ago. Spark were rather good with their response and tracked down the issue to being a bad update with outlook. Have you recently updated outlook? I remember the pain of having thousands of emails downloading constantly. Took microsoft at that point a week or two to get an update out.
i would love to bring this particular case to attention in relation to the OP's issue.
If you possibly still have a case number for this, would be keen to give it a skim..