l'envoi d'emails échoue avec gmail uniquement

Mon serveur debian envoie des emails à yahoo, hotmail mais pas à gmail !

tail /etc/log/mail.log

95.211.117.206], dsn=4.0.0, stat=Deferred: Connection timed out with 75gmail.com.
Nov 23 23:07:52 sitechat sendmail[1837]: tANM7qEk001837: from=www-data, size=1224, class=0, nrcpts=1, msgid=<201511232207.tANM7qEk001837@sitechat.com>, relay=www-data@localhost
Nov 23 23:07:52 sitechat sm-mta[1838]: tANM7q7O001838: from=<www-data@sitechat.com>, size=1476, class=0, nrcpts=1, msgid=<201511232207.tANM7qEk001837@sitechat.com>, proto=ESMTP, daemon=MTA-v4, relay=localhost.localdomain [127.0.0.1]
Nov 23 23:07:52 sitechat sendmail[1837]: tANM7qEk001837: to=hidden@gmail.com, ctladdr=www-data (33/33), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=31224, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (tANM7q7O001838 Message accepted for delivery)
Nov 23 23:07:52 sitechat sm-mta[1840]: STARTTLS=client, relay=gmail-smtp-in.l.google.com., version=TLSv1/SSLv3, verify=FAIL, cipher=ECDHE-RSA-AES128-GCM-SHA256, bits=128/128
Nov 23 23:07:52 sitechat sm-mta[1840]: tANM7q7O001838: to=<hidden@gmail.com>, ctladdr=<www-data@sitechat.com> (33/33), delay=00:00:00, xdelay=00:00:00, mailer=esmtp, pri=121476, relay=gmail-smtp-in.l.google.com. [IPv6:2a00:1450:400c:c02::1b], dsn=5.0.0, stat=Service unavailable
Nov 23 23:07:52 sitechat sm-mta[1840]: tANM7q7O001838: tANM7q7O001840: DSN: Service unavailable
Nov 23 23:07:53 sitechat sm-mta[1840]: tANM7q7O001840: to=<www-data@sitechat.com>, delay=00:00:01, xdelay=00:00:01, mailer=local, pri=30000, dsn=2.0.0, stat=Sent

J'ai configuré le reverse, changé le hostname, fait des tests avec test-mail.com où je reçois la note 9/10 (une très haute)

Que dois-je faire d'autre ?