Outlook in unable to connect to Office 365

Hi,

I am using Office 365.

I have two domains in my network with 2 ISPs.

ISP 1 is connected with AD-domain1 and ISP2 is connected with AD-domain2.

When i configure outlook 2013 in a machine with ISP1 + AD-domain1  it configures.

Also when i try to configure outlook 2013 with ISP1 + AD-Domain2 it works fine.

But when i try to configure outlook 2013 with ISP2 + AD-domain2 it gives me error message - "An encrypted connection to your mail server is not available, click next to start with an unencrypted connection"

it seems like it is not working with ISP2, what could be the issue. ANy help would be highly appreciated.

Thanks Chandan