ONE user can't send email to ONE external address

Hi All,

I have the following problem (driving me NUUUUUTS) : exchange 2010, not migrated, about 10 users, all tests done from OWA :

User A cannot send an email to User B outside of the domain, but User A can send email to anybody else and even more frustrating,any OTHER users of the same domain of A can ALSO send to B ! 

I created a 2nd database and move user A to it, didn't change anything. 

User A will receive the following error : 

IMCEAEX-_O=DOMAIN_ou=first+20administrative+20group_cn=Recipients_cn=Bkr26056554@DOMAIN.local

SBS2011.DOMAIN.local ([fe80::1332:1bfe:281c:b67%10]) with mapi id
 14.02.0318.004; Wed, 5 Dec 2012 18:49:32 +0100
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
From: User A <userA@accepted_domain.org>
To: "userB@external_domain"<IMCEAEX-_O=DOMAIN_ou=first+20administrative+20group_cn=Recipients_cn=Bkr26056554@DOMAIN.local>
Subject: test
Thread-Topic: test
Thread-Index: Ac3TEODi38oMZ+jkS7STRXD5WPDLrA==
Date: Wed, 5 Dec 2012 18:49:31 +0100
Message-ID: <48D9513965CEBA498E293CE96A135F2428DFBE21@SBS2011.DOMAIN.local>
Accept-Language: fr-FR, fr-CH, en-US
Content-Language: fr-FR
X-MS-Has-Attach:
X-MS-TNEF-Correlator: <48D9513965CEBA498E293CE96A135F2428DFBE21@SBS2011.DOMAIN.local>
MIME-Version: 1.0
X-Originating-IP: [84.253.20.227]
X-KSE-Antivirus-Interceptor-Info: scan successful
X-KSE-Antivirus-Info: Clean