Dear All, Current org have 1 centralize forest (Exchange hosted here) and few more separate forest (Users here). Understand users in separate forest to connect to the email from their Outlook. Users will require 2 logins, 1 to NT_Login and the other for the outlook. Q1. Correct me if Im wrong, to get SSO, we need to perform trust. Q2. Is there any alternative other than doing trust?

Identical usernames and passwords might work, but a trust is the only way that is recommended because it doesn't require users to maintain two usernames and passwords. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources

, Unfortunately, the credential is different for both forest.

In that case, no, without a trust you cannot have any form of SSO. If you don't have a trust, how does the user get authenticated in a way that doesn't expose the server to abuse. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources