I have a question. I just started working for a company that has started to deploy Office 365 to its users mostly for Exchange under a domain name like testdomain.com.

Internally we use a domain name such as corp.testdomain.com

What would be the best way to get SSO in a scenario like this - maybe added an additional UPN Suffix, AD Synchronization or ADFS?

Thanks for any insight.

There's no real SSO with Exchange Online, even when you have AD FS deployed the authentication will be proxied by the server to your AD FS farm and Outlook will ask for credentials. You can just use the "save credentials" checkbox to avoid the prompts, exactly like you would do with a non-federated account. So if you are only going to use Exchange, the only benefit you will see from SSO is when accessing OWA or the O365 portal.

That will change eventually when ADAL support is released.