E-mail addresses are converted by a hashing function into hash codes. An e-mail mass mailer that can have millions of addresses hash codes all addresses to which a message might be sent to form a first list. An entity that wishes to exclude delivery of a message to various e-mail addresses, hash codes such addresses to form a second list. Hash codes in the first list that are also in the second list are purged from the first list to yield a third list of hash codes, each of which represents an e-mail address to which a message may be sent.