Although phplist's Reply-to email address for system messages (message_replyto_address) was set to to noreply@[DOMAIN], I was having a problem where all system messages were appearing to come from the hosting account username. It turns out that my hosting provider was changing the "From:" address that I was passing to php's mail() function if I was using an email address that didn't exist. Changing it to a valid email address fixed the problem nicely.