Without knowing particulars, there are really two sets of headers in each e-mail: the kind that you can "see" -- these conform to RFC822; and those that you can't (usually) -- these are RFC821. The former are usually used to create the latter. It is the latter that is used though for actual transport. Depending upon the mail systems involved, you might have recipients that are enumerated in the RFC821 headers, but are NOT in the RFC822 headers. Some mail systems allow a user to do that very easily by use of a BCC: field when they are composing the e-mail. Obviously other mail transport mechanisms such as LISTSERV use a similiar vehicle when your subcription distribution options are SHORT (and others).