feedmail-deduce-envelope-from

feedmail-deduce-envelope-from is a variable defined in `feedmail.el'.
Its value is

t

Documentation:
If non-nil, deduce message envelope "from" from header From: or Sender:.
In other words, if there is a Sender: header in the message, temporarily
change the value of `user-mail-address' to be the same while the message
is being sent. If there is no Sender: header, use the From: header,
if any. Address values are taken from the actual message just before
it is sent, and the process is independent of the values of
`feedmail-from-line' and/or `feedmail-sender-line'.

There are many and good reasons for having the message header
From:/Sender: be different from the message envelope "from"
information. However, for most people and for most circumstances, it
is usual for them to be the same (this is probably especially true for
the case where the user doesn't understand the difference between the
two in the first place).

The idea behind this feature is that you can have everything set up
some normal way for yourself. If for some reason you want to send a
message with another From: line, you can just type it at the top of
the message, and feedmail will take care of "fixing up" the envelope
"from". This only works for mail senders which make use of
`user-mail-address' as the envelope "from" value. For some mail
senders (e.g., feedmail-buffer-to-bin-mail), there is no simple way to
influence what they will use as the envelope.

You can customize this variable.