On Thu, Apr 26, 2018 at 02:11:44PM +0200, to...@tuxteam.de wrote: > That's right, mbox uses this one as marker. That's why a "naked" From at > the beginning of a line, like this one: > > From there and where > > gets "escaped" by prepending something (I think this is done by the > mail delivery agent).
> I'm here "on" mbox, but I'd expect to have this line also in other > civilised mail storages (perhaps excluding Exchange/Outlook, but hey > do they count as civilised?) This piece of information (to whom errors should be sent) is called the "envelope sender". It's part of the SMTP conversation between the sending MTA and the receiving MTA. In mbox formats, the "From " header line may include the envelope sender, but I don't think it's guaranteed. <http://jdebp.eu./FGA/mail-mbox-formats.html> describes it as a "convention". In other formats, there is no "From " line, because messages are separated in a different way. E.g. Maildir stores one message per file, and MMDF uses 4 consecutive Ctrl-A characters within a single file -- see <http://www.tin.org/bin/man.cgi?section=5&topic=mmdf>. Which is NOT to say that there is no record of the envelope sender in these other systems. In qmail's Maildir, for example, the envelope sender is written in a "Return-Path:" header line (by qmail-local -- see <http://www.qmail.org/man/man8/qmail-local.html>). For other mail systems and mailbox formats, I would suggest consulting your documentation. Some may not store it at all. Some may copy Maildir's conventions. Who knows what the others do....