Who sets 'Return-Path'?

[Ben Beuchler]

I'm sure this is a basic question, but who controls the 'Return-Path'
header?  The MTA or MUA?

Thanks,
Ben

-- 
The spectre of a polity controlled by the fads and whims of voters who
actually believe that there are significant differences between Bud Lite
and Miller Lite, and who think that professional wrestling is for real, is
naturally alarming to people who don't.
                -- Neal Stephenson