Winfile On All Outbound Emails

I currently have a user running Windows XP SVC 2, MS Outlook 2003 who keeps encountering recipients unable to open his email due to winfile errors. I have verified the emails go out in plain text and followed all steps by Microsoft. All patches are current. Any idea’s? I can send from my workstation but am running Outlook 2007 but do not plan to roll that out to end users.