[Exim] (no subject)

Hello exim users,

I'm quite newer in using exim.
I have a linux server (Debian) running exim.
It was working well for many years but I recently changed it's subnet mask and
moved it into a new DMZ (firewall replacement).
Since this change, e-mail are not going out :
I can receive e-mail from any domain name
I can send e-mail to people from my own domain (bm-rennes.fr)
But I can't send e-mail to other domain names. The error message is (in
french...):

"Impossible d'envoyer le message car l'un des destinataires a été refusé par le
serveur. L'adresse d'email refusée était 'informatique-nec@agglo-
rennesmetropole.fr'. Objet 'objet', Compte : 'Compte MF Ballé',
Serveur : '212.234.217.177', Protocole : SMTP, Réponse du serveur : '550
relaying to <informatique-nec@???> prohibited by
administrator', Port : 25, Sécurisé (SSL) : Non, Erreur de serveur : 550,
Numéro d'erreur : 0x800CCC79"

Do you have any idea for analysing this issue ?
Many thanks

Cordially,
Forveille Thierry
Rennes Metropole
Finances & Systèmes d'information