The home realm URL issue is introduced when we attempt to add another active directory or claims provider for a domain outside our cloud domain to create single sign on. (SSO). Microsoft Internet Explorer has the ability to support this and we have no issues connecting via the browser.
However, Microsoft Outlook as well as third-party applications currently are not ready to support multiple home realm URLs. This issue is becoming more and more apparent as customers
move to more cloud based services for requiring SSO.
Microsoft has addressed this issue for the outlook client, by creating the following registry key:
under the registry entry:
This Microsoft article explains this more in detail here:
This can be a gotcha for third-party ISV’s that are not coded to support multiple home realm domains.