Tony Finch wrote:
The envelope is for consumption by transport-level agents, and is
discarded on final delivery. (Therefore IMAP and POP don't provide access
to it because it isn't there.
Interestingly enough, there are extensions for accessing envelope FROM
from IMAP, as well as checking if the received message was authenticated
using SMTP AUTH.
) This means that user agents can't see the
envelope. Any feature that depends on envelope information to work (e.g.
DSNs) must therefore be implemented by a transport agent. Any feature that
must be implemented by a user agent (e.g. MDNs) must be signalled in the
header not the envelope.