Outlook asking for credential continuously

I have Exchange 2013 CU2 environment. Users are using outlook 2007 & 2010. We had AD integrated DNS servers listed 
in the primary and secondary DNS in Network adapter settings of all Exchange Servers. Today one of our internet link
went down. I changed gateway & secondary DNS (entered external DNS as secondary DNS) on all exchange servers. Emails even inside the organization stopped.

I changed back the secondary DNS & Gateway after resumption of internet services. Email started to flow. But half of our
organization is facing continous credential prompt. Affected users are using both 2007 & 2010 versions.
I could not verify exactly if affected users are from a certain database.

I deleted and recreated outlook profile  for couple of users and issue got resolved for them but I can't do it for 600 users.
On users Outlook "negotiate authentication' is checked and 'Always prompt for login credentials' is unchecked.

I unchecked "Connect Exchange using Http" option but upon restarting, outlook showed 'Disconnected' though Users are 
not using Outlook Anywhere. 

I remember previously (one month ago) one of user facing this credential prompt issue. I changed the IE proxy to a 
different ISA server in our organization, resulting in resolution of the problem.

Can any one please help.