Question : Difference between return-path and envelope sender address

Arent both set by "mail from:"?
Whats the difference who sets em and how they are handled?

Answer : Difference between return-path and envelope sender address

The Return-path (where bouncebacks are sent) is generally copied from the Mail From command, but for compatibility with all mail systems which may handle the message further down the line which may not be as intelligent, the Return-path address is also set in the header of the message.

See http://en.wikipedia.org/wiki/Bounce_message

-tigermatt