On 2019-06-08 00:41, Cameron Simpson wrote:
Well, the From_ isn't just a delimiter for mbox lines, it also historically contains the envelope address from the mail system - the address used for this delivery (versus whatever may be in the headers).
It's the envelope sender address, who SMTP said the message is from (possibly different from header From:). That's redundant these days. The envelope sender address is also in Return-Path:. That header probably didn't exist when mbox was invented, in the 1970s, but it's been around for a long time. Oh look, Return-Path: is in RFC 822, published in 1982. Its syntax is a little different in RFC 2822, 2001.
Header fields are lines beginning with a field name, followed by a colon (":"), followed by a field body, and terminated by CRLF. A field name MUST be composed of printable US-ASCII characters (i.e., characters that have values between 33 and 126, inclusive), except colon.
Well, there's the thing about registering header names with IANA, or starting unregistered ones with "X-". But it's a fine point in this situation.
