For inbound emails we want to use:
When centralized mail transport is disabled (default configuration), incoming Internet messages are routed as follows in a hybrid deployment. So all inbound email will route to office 365 before it works out which mail store to deliver email. This is fine as we want to move all mailboxes to online.
OK So for the outbound we will use this for now as the on prem addresses will be fine to go directly outbound and after this all accounts will be moved to the EX Online org at which time they will use the online servers to relay email:
Don’t enable centralized mail transport Selected by default in the Hybrid Configuration wizard, this option routes outbound messages sent from the Exchange Online organization directly to the Internet. Use this option if you don't need to apply any on-premises compliance policies or other processing rules to messages that are sent from recipients in the Exchange Online organization.
I assume as I am accepting the default routes as pre your document that things will work after I change the DNS records previously mentioned. What I want to make sure of however is that when the DNS changes that I am not going to have any problems. Is there a sure fire way to ensure this is going to work? I need a test that proves that all email will go in through EX online into either online mailboxes or EOP mailboxes.
I can see my telnet test has sort of proven this but as I did not get the email into the recipient email account due to the spam filtering (I think). If I send a message from an online mailbox to a EOP mailbox will this prove that the route is working?