To determine where a message is to go, the following algorithm is used by default.
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
A followup is a response sent via news. The following headers (listed in order of precedence) determine where the response is to be sent: