This field is present only if the submitter manually enters a From line. It is intended to record the entity responsible for submitting the message to the network. It should be verified by the software at the submitting host.
For example, if John Smith is visiting CCA and wishes to post a message to the network, using friend Sarah Jones' account, the message might read:
From: smith@ucbvax.Berkeley.EDU (John Smith)
Sender: jones@cca.COM (Sarah Jones)
If a gateway program enters a mail message into the network at host unix.SRI.COM, the lines might read:
The primary purpose of this field is to be able to track down messages to determine how they were entered into the network. The full name may be optionally given, in parentheses, as in the From line.
RFC1036: [Source:"RFC-1036"] [Last Changed:December 1987] [Copyright: 1987 M. Horton, R. Adams]
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Up to RFC1036 2.2. Optional Headers
RFC1036 2.1.1. From