The is error is caused by a number of different reasons but most often a simple reboot of the computer to refresh Outlook will fix this.
If this happens on download it means there may be an email stuck in your outbox. You can fix this simply by logging into your web based email client that comes standard with your hosting plan and check the email and remove any strange files. This often is an error caused by Outlooks Metabase not knowing what to do and how to process what ever your asking it to do.
Some other fixes include patching your Microsoft Office Software