Can you not send it as a .pdf or ,eps, which will retain both vector and raster info? Unless I am mistaken, most(?) printers should be able to use either format.
Other than that, when you are exporting your raster (print) file, make sure that none of your parameters (aspect ratio) are changing. Sometimes, if you change your resolution to email, the dimensions change too, so check your export settings.
Good luck,
Mark