postfix:: relayhost :: changer le domaine de l'expediteur

Hi,
je suis sur Suse 9.1
fetchmail + postfix + cyrus
pour envoyer mes mails j'ai acces à un serveur exchange que j'utilise comme relay. le serveur exchange n'est pas le mien et je ne peu faire aucun changement de configuration dessus.
celui là n'expedie que les mails dont l'expedieur à une adresse: @mon_domain.com
lorsque je fais un .forward à des adresses exterieur, les mails forwardés ont comme expedieur nimporteki@nimportekoi.xx, donc le relay-host ne les accepte pas et ces mails disparaissent. que faire?
y a t il une possibilité de modifier mon main.cf pour masquer le vrai expediteur de facon a ce que le relay-host croit que les mails proviennent de mon_domain.com?
merci.
Roy

p.s.: ma main.cf:
---------------



alias_maps = hash:/etc/aliases

allow_percent_hack = yes

append_at_myorigin = yes

append_dot_mydomain = yes

canonical_maps = hash:/etc/postfix/canonical

command_directory = /usr/sbin

config_directory = /etc/postfix

daemon_directory = /usr/lib/postfix

debug_peer_level = 2

default_transport = smtp

defer_transports =

disable_dns_lookups = no

empty_address_recipient = MAILER-DAEMON

inet_interfaces = all

local_recipient_maps =

mail_owner = postfix

mail_spool_directory = /var/mail

mailbox_command = /usr/lib/cyrus/bin/deliver

mailbox_size_limit = 51200000

mailbox_transport = cyrus

mailq_path = /usr/bin/mailq

manpage_directory = /usr/share/man

masquerade_classes = envelope_sender, header_sender, header_recipient

masquerade_domains = $myorigin

masquerade_exceptions = root

message_size_limit = 10240000

mydestination = localhost, mon-domaine.com, localhost.mon_domain.com

mydomain = mon_domain.com

myhostname = my-server

mynetworks_style = class

myorigin = $mydomain

newaliases_path = /usr/bin/newaliases

queue_directory = /var/spool/postfix

readme_directory = /usr/share/doc/packages/postfix/README_FILES

relayhost = 10.2.1.12

relocated_maps = hash:/etc/postfix/relocated

sample_directory = /usr/share/doc/packages/postfix/samples

sender_canonical_maps = hash:/etc/postfix/sender_canonical

sendmail_path = /usr/sbin/sendmail

setgid_group = maildrop

smtp_sasl_auth_enable = no

smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

smtp_sasl_security_options =

smtpd_client_restrictions =

smtpd_helo_required = no

smtpd_helo_restrictions =

smtpd_sasl_auth_enable = no

smtpd_sender_restrictions =

smtpd_use_tls = no

strict_rfc821_envelopes = no

transport_maps = hash:/etc/postfix/transport

unknown_local_recipient_reject_code = 450