We have 3 clients on a network that started getting this error when downloading email from a non-gmail account (fetching using pop3). I've done some research and found many posts on it being a message that was >20Mb which is a limit for gmail downloading messages.
However the user can login to the remote server using webmail, and there are no email messages that are >1Mb let alone 20Mb! Also, we've setup Outlook 2007 as a test and it downloaded all the messages just fine and again shows no messages that are large.
I've also looked at her filters and she doesn't use them, so that's not the problem.