Exchange 2013 : 550 5.1.1 User unknown for many recipients except just one recipient address is wrong

Hello,

We have a strange error with our Exchange 2013 Server (CU13).

When an external user sends mail to 2 recipients which have same domain (samedomain.fr for example), if an address is wrong, the sender receives an error (see below) says 2 address are unknows. 

                   The mail system

<test3@samedomain.fr>: host mail.samedomain.fr[X.X.X.X] said: 550 5.1.1 User
    unknown (in reply to end of DATA command)

<toto@samedomain.fr>: host mail.samedomain.fr[X.X.X.X] said: 550 5.1.1 User
    unknown (in reply to end of DATA command)

Any ideas ?

Thanks for help.

Benoît