Exchange Online and Azure AD Connect

Hi Ruairidh,

Microsoft recommends that you have an Exchange on Premise to configure mail settings for users, and if you uninstall Exchange on-prem you can't setup Email Address Policies or additional proxy addresses.

So Its much better to leave at least one Hybrid Exchange server on-premises even after all mailboxes have been migrated to Office 365, to allow easily manage mailboxes from a single console. Remember that since the source of authority is the on-premises AD (because AAD Connect), many changes need to be made on-premises. If there is no longer an Exchange server to manage and update mail attributes, you have to turn to 3rd party tools or work with ADSIEDIT.

In your scenario you must to do merge with Office 365 account with an on-premises AD account and to do a soft match between objects and values.

Once you will finish the merging you will be able to configure Seamless SSO

Note:

For Office 365 plans you get a free Exchange Server Hybrid Key: http://aka.ms/hybridkey

The Exchange On-Premises is for manage without any configuration and some settings and components need to disable such client access etc.

Eli.