Jnet appends a special Received: line to all messages passed to BN_SLAVE. This Received: line contains both recipient and sender addresses for the message. No other envelope information is provided by Jnet; in particular, no envelope From: information is available; the Received: line only gives the sender of the message, which is often a mailer or list server of some kind and is not actually a From: address.
BN_SLAVE uses the recipient specified in the Received: line to construct the envelope To: address. However, the envelope From: address is constructed using a system similar to the one used to construct VMS MAIL From: addresses --- the beginning of the message is parsed as an RFC 822 header and an attempt is made to use the fields from this header. The fields that are used, in order of decreasing priority, are: