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.)