Messages sent via an ezmlm mailing list come ‘From:’ the original sender. For many lists, it is appropriate to discuss privately with the original sender, then post a follow-up message with e.g. a solution to the list. In this case, just “Reply” to the sender. Without this behavior two things may happen: First, a subscriber may intend to reply to the original sender only, but sends a message to the entire list. This can be very embarrassing. Second, misconfigured subscriber programs sometimes send “John is on vacation” messages back to the ‘From:’ address. The ezmlm setup allows these to go to the original sender (a minor nuisance) rather than to the list (a major nuisance with the potential for mail loops if the subscriber software is severely misconfigured).
In other cases, you may want to send the reply to all subscribers. The easiest is to type in the list address by hand or to use the “Reply-to-all” function of your mail reader. If you do this, delete your address as well as the original sender's address so that the message goes to the list only. Since messages reach subscribers within minutes, it is usually superfluous and sometimes annoying to also send a reply directly to the sender as well.