EDIT : Solution trouvé, j'avais _domainkey.domain.tld et pas mail._domainkey.domain.tld 🙂
alors petit souci pour moi ( pas encore eu le temps de lire tout le fil de discussion )
c'est au niveau du Dkim, donc :
mon serveur : hostname : ohmna fqdn: ohmna.domain.tld
tuto suivi à la lettre, mais sur les validateur dkim (mail-tester, dkim validator)
il me dit que j'ai 2 signatures dkim cf un log dkim de dkimvalidator :
DKIM Information:
DKIM Signature
Message contains this DKIM Signature:
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=domain.tld; s=mail;
t=1441113213; bh=CCfBVnFKHse5bF7UOVhjJx6+VRLne7CKGYrQtB5oGTs=;
h=Date:From:Subject:To:From;
b=nsRbrSG8ngjFAwrfJGCRd79dzJcdyGmxRHDmz4Ic1w6Bfzu2b0f9tQQZufJRWXKJR
4ISItIyAfl98CG3NrZazCu6d+qin+NPyi2uLbWGa8PlchDKc2uecMm1Vc+1OQQ9+BV
Ec4u2iUpB4552JkG8s+xHxOQ28bC5lvx84MHLRws=
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=domain.tld; s=mail;
t=1441113211; bh=CCfBVnFKHse5bF7UOVhjJx6+VRLne7CKGYrQtB5oGTs=;
h=Date:From:Subject:To:From;
b=FguAgAVbLtv6JaExAURQkmXky28JYCoAdvKE7kaB33W/lxQRqgGhAzEq1ocAMWbaC
P61i4XJCHALOIrizti0nuG2KBs197DD82mvC9szx6PUEypvxPEDcqN/w+ReXBJj8I9
xy4SED9M+RaGLw16VmClwIhsdwt+rWzBQ5LvVa/A=
Signature Information:
v= Version: 1
a= Algorithm: rsa-sha256
c= Method: relaxed/simple
d= Domain: domain.tld
s= Selector: mail
q= Protocol:
bh= CCfBVnFKHse5bF7UOVhjJx6+VRLne7CKGYrQtB5oGTs=
h= Signed Headers: Date:From:Subject:To:From
b= Data: nsRbrSG8ngjFAwrfJGCRd79dzJcdyGmxRHDmz4Ic1w6Bfzu2b0f9tQQZufJRWXKJR
4ISItIyAfl98CG3NrZazCu6d+qin+NPyi2uLbWGa8PlchDKc2uecMm1Vc+1OQQ9+BV
Ec4u2iUpB4552JkG8s+xHxOQ28bC5lvx84MHLRws=
Public Key DNS Lookup
Building DNS Query for mail._domainkey.domain.tld
Retrieved this publickey from DNS:
Validating Signature
result = invalid
Details: public key: not available
voila mon enregistrement dns ( anonymiser

)
$TTL 3600
@ IN SOA dns15.ovh.net. tech.ovh.net. (2015090109 86400 3600 3600000 300)
IN NS dns15.ovh.net.
IN NS ns15.ovh.net.
IN MX 1 domain.tld.
IN A ip.de.mon.serveur
600 IN TXT "v=spf1 a mx ip4:ip.de.mon.serveur ~all"
_autodiscover._tcp IN SRV 0 0 443 domain.tld.
_dmarc IN TXT "v=DMARC1; p=reject; rua=mailto:postmaster@domain.tld; ruf=mailto:admin@domain.tld; adkim=s; aspf=s; pct=100; rf=afrf; sp=reject"
_domainkey IN TXT "v=DKIM1; k=rsa; p=Cle Dkim"
_imaps._tcp IN SRV 0 0 993 domain.tld.
_submission._tcp IN SRV 0 0 587 domain.tld.
autoconfig IN CNAME domain.tld.
ohmna IN A ip.de.mon.serveur
postfixadmin IN CNAME ohmna.domain.tld.
webmail IN A ip.de.mon.serveur
www IN CNAME domain.tld.
je vois pas ou j'ai un soucis