I had this with Trend Micro spam filter. After having pressed the report spam, I received this error continuously. I found that an email in the drafts folder which the Trend micro software had created. In the mail was an attachment containing the original spam mail. This spam mail had erroneous data causing the send failure. I couldn't delete it as it was protected. So I opened it and closed it. This freed it up and I could delete it. Having removed the faulty mail, my sending problem went away.
david
October 2009