For an acquisition strategy we may have a tenant were 100 mailboxes will be in the cloud and 1500 will be on-premises.
Normally this is no big deal, right?
But in this case we only plan to DirSync (AD Connect) the 100 AD Users with the mailboxes in the cloud.
Inbound mail will hit on-premises and that should work fine.
My question involves what we will have to do to get internal email to travel from the cloud mailboxes to the on-premises mailboxes? Will that require conditional based routing with a transport rule or will the hybrid created connectors be enough... or must we sync all AD Users with a mailbox.
Guessing that you have MX record already configured.....
If you have dirsync then you can see all users in portal but your on-prem users will not have any licensed assigned.
If your domain name is same in cloud and on-prem so the GAL will remain same...