Return-Path vs. unsubscribe and X-CiviMail-Bounce
Turns out it was a problem with exim, which was apparently changing the headers civi was sending.

After working with our host they were able to adjust exim.conf to make it work.  Here's what they did which may or may not be completely correct, and may or may not solve others' problems, but it' a start

At the very top of exim.conf add 

local_from_check=false
untrusted_set_sender = true
local_sender_retain = true

Here's the reference: http://www.exim.org/exim-html-current/doc/html/spec_html/ch-main_configuration.html

"If local_from_check is set false, the From: header check is disabled, and no Sender: header is ever added. If, in addition, you want to retain Sender: header lines supplied by untrusted users, you must also set local_sender_retain to be true.

These options affect only the header lines in the message. The envelope sender is still forced to be the login id at the qualify domain unless untrusted_set_sender permits the user to supply an envelope sender."