- Envelope Recipient
- Mail is sent to the envelope recipient. The envelope
recipient may or may not appear in the To or Cc mail
headers. Sendmail is prone to destroying the envelope recipient
information but it appears semi-reliably in X-Envelope-To.
- Envelope Sender
- Mail is sent with an envelope sender. This address is used for
bounce messages. The address may or may not appear in the
From or Sender mail headers. The envelope sender
should appear in a Return-Path header (aka Unix
From line in a Mailbox).
- Mail Transport Agent (MTA)
- A MTA is responsible for transporting mail. The one we're dealing
with here is sendmail. Sendmail
is wonderful (Internet mail was built with it) but it is urgently
needing to be put out to pasture now.
qmail is a superior MTA.
- Mail User Agent (MUA)
- Mail user agents are the programs users use to read and send
mail. mdforward is actually a MUA, although it usually just hands
mail back to the local MTA for forwarding.
- Sendmail sometimes puts the envelope recipient here. Sometimes
what is here is not sensible though; it can even be another user!
(I've seen pathnames here too ... sendmail is one weird MTA.)