I have been having a grapple with this - I use Pluto and POPstar.
I have found that it was Pluto in its User list that was making my stuff
sent through BT look as though it was being allegedly sent through
free.fr.
I had forgotten that it was that - and for me the default top entry in the
user list - that was defining the From: in the headers.
So I am now sending mail through BT using my (otherwise almost unused) BT
address as the user e-mail at the BT domain - which now matches my IP
address range (naturally), but have put the user name as my prefered one,
which some of you will already know is with a French provider.
The name "JohnBT" still over-rid (over-rided? was over-riding?) my
preferred (French) name I still wished to present-as until I deleted that
other pre-existing user-entry, which was "JohnBT". Where there was
duplication, Pluto seemed to pick-up the name from the last one
A complication: I have my own GMail set-up to copy to Pluto, and had a
filter to redirect incoming to a GMail folder simply using the GMail
address in the header. I've now changed this to "Not" include it in the
author field, and now stuff I send deliberately to my French user arrives
in the correct box. Useful for testing this as well as "belt & braces"
when other file transfer methods between OSs seem overkill, though I do use
FTP a lot as well as LanMan.
I'll need to keep an eye on that filter to make sure it works as intended.
Now, as I have my reply-to set to my French provider/address, replies still
arrive there as they always did.
BT seems quite happy to send stuff through the Free France STMP server
whilst maintaining its origins as BT in its UK address range. So fingers
crossed that I'm sorted.
So, in summary, in Pluto Users the top entry (if you're out of the habit of
setting it every time yourself) should have a UserID (e-mail first bit) and
domain that agrees with your provider, though the next field, Name, is your
choice, as is the Signature file, which is where you might put your
(different?) ReplyTo.
So if that helps anyone else, let me know! Or if I'm not as clear as I
might wish to be, take me to task.
Best wishes,
John