How to set envelope sender (MAIL FROM) using exim

[Longman]

I can use -a "From: foo at bar" as an option to /usr/bin/mail but the
return-path (envelope sender) on outbound emails is of the form
user at mailname.  For some reason a system I mail ignores the From address
and wants to reply to the return-path address despite there being no
Sender header present and the From being valid and deliverable.

What solutions are there?