Ken's right (and I mislead Parker): MPEG Streamclip can't output MPEG-2. It reads MPEG-2 files with such aplomb and does several other conversions so readily, that I assumed it could convert to MPEG-2 format.
Try MPEG StreamClip It should do exactly what you want. Assuming you want MPEG-2 output (and not MPEG-1), you must also purchase Apple QuickTime Pro ($20), or install QuickTime Alternative (free).
I used Mailwasher Pro for a year or two, since I get about 100 spam per day. It seemed great at first, but I eventually dropped it for a new solution. I now route all my email through Gmail to serve as my spam filter. Gmail's not perfect, but it's much simpler and faster than MailwasherPro. And...