There is one other esoteric force at work ... normal list distribution
traffic is "handled" by the LISTSERV(R) Distribute Backbone, where as
command activity is more like (Ben's and Roger's explanations
notwithstanding) "normal" e-mail.

This means that different mail transfer agents [may] deal with the two
kinds of traffic, and their associated filter/a-v/spam rules.

Pete-Weiss@PennState