>In general, if a user has something other than FULLHDR set, does that restrict
>what headers get propagated?I'm thinking in particular about Return-Receipt-To
>and/or X-Acknowledge-To (i.e. would they be filtered?)
 
Yes. SHORTHDR (the default) passes a minimal set of headers (not
much more than To: From: Reply-to: Subject:) and strips
everything else.
 
Note that Return-Receipt-To: is a sendmail-ism, and is not
reliable in any sense of the word (most other mailers just ignore
it).