Hello Experts. I have exchange 2013 CU19 in my environment with AD 2012 R2. We are thinking of migrating mailboxes to Office365. I am thinking of leveraging our ADFS 3.0 infrastructure for SSO. I know when a users mailbox is migrated to O365, and after the cut-over, they are prompted to restart their Outlook, after the first restart, Outlook also prompts them for credentials. this is normal behaviour as the user now needs to authenticate against Exchange online.
With ADFS, even this first credentials prompt for the user should not appear, and ADFS will provide full Seamless SSO. My question is, we have multiple accepted domains in our on-prem Exchange, and for reasons, we cannot add/ verify all the domains at once in O365. so if a user on-prem has delegation access to a migrated users mailbox (that exists in O365), they will also get a prompt to restart their outlook, and will receive a cred prompt from Outlook the first time...will ADFS be able to authenticate the on-prem user aswell to Exchange online? or will we be required to add all of our accepted domains to O365/ Azure for ADFS to authenticate both on-prem and cloud users?