Route internal mail through on prem and external through O365

[Andrew O'Young]

We have a customer who has a Hybrid O365 environment.  All mailboxes are still on-premises. 

Due to frequent power outages we need to migrate their email to the cloud. 

The problem with this is that they have incredibly low bandwidth. 

What we would like to see is all internal mail route via the on-premises server (i.e. never actually go out to the internet) while outgoing email goes to the internet. 

We would also like to see mail go to 365 if the on-premises server is not reachable.  

[#2]

need A bit more information on the setup. But if your onprem is still hosting the mailboxes unless you have some kind of connector forcing all mail out the internal mail will route onprem by default.

To satisfy if your talking about inbound mail coming in and routing to 365 then you need to setup 365 as backup mx record so it routes there when you are down. Then when you come back online the queued yo messages can deliver via connector.

Other factors are unknown here as what does security scanning of messages? If that’s in 365 then all your inbound and possibly outbound email should be going there regardless. But internal mail should just route internal by default.

[#3]

Thanks Christopher, I have amended the opening post to hopefully clarify some details.