Autodiscover Office365 - TMG - The Connection was refused

Hello,

our company is using Office365 Exchange.

When I want to add an account to the Outlook desktop client, which is behind TMG firewall, it takes about 30 minutes when Outlook prompts for password again. Once I enter, account is provisioned.

When I do the same on the same PC, without TMG, it asks me for these credentials within 10 seconds.

I have All Outbound enabled - so no traffic is blocked from internal to external network.

I see an error on TMG - "The Connection was refused"

TMG is in factory defaults - tried on 2 instances.

DNS and other stuff is set correctly - validated several times on O365 portal. All DNS entries are available for PC behind the TMG.

There was exactly same problem with no resolution:

Can you please advise?

Thanks