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If using OWA I can access my mailbox via https://www.mydomain.fr/owa or https://servername.mydomain.fr/owa, I have registered both addresses possible.
But none of them are allowed in BIS.
In BIS I enter :
URL : https://www.mydomain.fr/owa
USER : DOMAIN\user or just user
PASS : xxxxx
MAIL : firstname.lastname@example.org
MAILBOX : I tried blank or username, but don't work