followed steps from here : http://stackoverflow.com/questions/10359437/sendmail-how-to-configure-sendmail-on-ubuntu

AuthInfo:mail.example.com "U:xxxrxxx" "I:no-reply@example.com" "P:coxxxox)"

in sendmail.mc file added : Masquerade
('SMART_HOST','mail.example.com')dnl
define('confAUTH_MECHANISMS', 'EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
FEATURE(`authinfo',`hash /etc/mail/auth/client-info')dnl
MASQUERADE_AS(example.com)dnl
FEATURE(`allmasquerade')dnl
FEATURE(`masquerade_envelope')dnl

then :
$m4 sendmail.mc > sendmail.cf
$service sendmail restart

but the DS line in sendmail.cf is still empty

--------------- Mail.log
Sep 14 16:39:53 xdgs sendmail[5460]: r8EB9rjw005460: from=root, size=39, class=0, nrcpts=1, msgid=<201309141109.r8EB9rjw005460@xdgs.localdomain>, relay=root@localhost
Sep 14 16:39:54 xdgs sm-mta[5461]: r8EB9s7o005461: from=<root@xdgs.localdomain>, size=324, class=0, nrcpts=1, msgid=<201309141109.r8EB9rjw005460@xdgs.localdomain>, proto=ESMTP, daemon=MTA-v4, relay=xdgs.localdomain [127.0.0.1]
Sep 14 16:39:54 xdgs sendmail[5460]: r8EB9rjw005460: to=wothwhere@gmail.com, ctladdr=root (0/0), delay=00:00:01, xdelay=00:00:00, mailer=relay, pri=30039, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (r8EB9s7o005461 Message accepted for delivery)
Sep 14 16:40:01 xdgs sm-mta[5463]: STARTTLS=client, relay=gmail-smtp-in.l.google.com., version=TLSv1/SSLv3, verify=FAIL, cipher=ECDHE-RSA-RC4-SHA, bits=128/128
Sep 14 16:40:06 xdgs sm-mta[5463]: r8EB9s7o005461: to=<wothwhere@gmail.com>, ctladdr=<root@xdgs.localdomain> (0/0), delay=00:00:12, xdelay=00:00:12, mailer=esmtp, pri=120324, relay=gmail-smtp-in.l.google.com. [74.125.129.26], dsn=2.0.0, stat=Sent (OK 1379157006 ua7si14752266pac.59 - gsmtp)
---------------