Internet addresses appended

We have one GW6SP2 domain and P.O. Sometimes, when a person sends
mail
from outside GW and has addressed multiple local GW users, there are
problems with the "reply to all" option. We're finding that certain
local
users will have the internet domain of the sender appended to the
local
E-Mail address. For instance, a message sent from Harry@zwiff.com that
has
been addressed to toleary@dyson.org and Freddy@dyson.org (Our GW
internet
domain) and has been responded to with the "Reply to all" option, will
have
at least one of the "dyson.org" adressees bounced back with an
"unknown
user" message because "whomever"@dyson.org looks like
"whomever"@zwiff.com.
If I look at the "reply to all" recipients before sending the
reply out
GW6 lets you get a snapshot of the addressees details. The details of
the
bounced person will show "E-Mail Address:
"Whomever.PO1.DYSON@zwiff.com,
appending the senders internet domain to the local GW P.O..

Have any of you seen this sort of thing that could give me an idea
on
how to fix it?
T.I.A.

Tim O'Leary