A method of sorting mail according to the invention includes the steps of receiving a plurality of first mail pieces of unknown origin in a first apparatus for bulk processing of the mail pieces, processing the mail pieces in the first apparatus by one of testing the mail pieces to determine if a potentially dangerous microorganism is present, sterilizing the mail pieces to destroy microorganisms on mail pieces, or both, and then if no contamination is detected or if sterilization is carried out, combining first the mail pieces at a later stage of postal processing with second mail pieces from senders identified as having a lesser risk of contamination than the first mail pieces of unknown origin. The invention further provides mail contaminant detection and sterilization systems suitable for use in such a method.