A reply is when you want to respond just to the person who sent the message via mail. There will only be one recipient. To determine who the recipient will be, the following headers are consulted, in turn:

A wide reply is a mail response that includes all entities mentioned in the message you are responded to. All mailboxes from the following headers will be concatenated to form the outgoing To/Cc headers:

If a Mail-Copies-To header is present, it will also be included in the list of mailboxes. If this header is `never', that means that the From (or Reply-To) mailbox will be suppressed.

A followup is a response sent via news. The following headers (listed in order of precedence) determine where the response is to be sent:

If a Mail-Copies-To header is present, it will be used as the basis of the new Cc header, except if this header is `never'.