A copy of this field is added by each transport service that relays the message. The information in the field can be quite useful for tracing transport problems.
The names of the sending and receiving hosts and time-of-receipt may be specified. The "via" parameter may be used, to indicate what physical mechanism the message was sent over, such as Arpanet or Phonenet, and the "with" parameter may be used to indicate the mail-, or connection-, level protocol that was used, such as the SMTP mail protocol, or X.25 transport protocol.
Some transport services queue mail; the internal message identifier that is assigned to the message may be noted, using the "id" parameter. When the sending host uses a destination address specification that the receiving host reinterprets, by expansion or transformation, the receiving host may wish to record the original specification, using the "for" parameter. For example, when a copy of mail is sent to the member of a distribution list, this parameter may be used to record the original address that was used to specify the list.