Welcome to Seven Forums lewis shaw. You are using Windows Live Mail (WLM)? The error you received is often caused by the email scanning part of various antivirus programs. Try disabling your AV program momentarily to see if you can send an attachment.
What AV program are you using, and how large is the attachment you are trying to send? Or does any size attachment cause this error? Did this problem just begin? If so, you might try a System Restore point from before the problem began. A Guy