I recall patching majordomo to sort the huge subscriber list (wanting to know a court's actions as they happened) by reverse string, to bring like domains together, before splitting it into chunks of N subscribers and handing those individually to sendmail. Before that, sendmail would be given one email with a huge subscriber list and plod through the recipients, without sorting them so it contacted the same SMTP server many times, and one unresponsive peer would stall delivery for all that followed.