Some people must pay for the time that they are connected to their e-mail server. Others must pay for all data they receive by the byte.
Discussion is the name of the game.
Just use what's relevant. Occasionally, message delivery to an individual list member fails, for one reason or another. Regardless of the reason, non-delivery of list messages to any member is a real list management problem that must be resolved immediately. The member's ISP has accidentally misconfigured its e-mail server. A network outage has occurred.