Tu as le choix : exim, sendmail, qmail, postfix... leurs docs respectives indiquent comment mettre en place une passerelle, et la doc de spamassassin t'indiquera comment greffer ce dernier sur le relay ainsi créé.
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"The marketing guys said the HP-35 would be a failure because it was too small, and then we couldn't make them fast enough to meet the demand. The marketing folks don't know everything." - Bill Hewlett