Exchange Hybrid mailbox availability and licensing stuff?

[Marko Farkas]

Hello,

I am trying to deploy exchange hybrid configuration. I have an on-premises Exchange 2016 server and a limited number of Office 365 licenses. I have questions about this (we can consider that MX record is pointing to Exchange Online Protection service, so incoming mail goes trough online service first, and that centralized mail flow was not enabled in hybrid configuration wizard):

1. Can hybrid configuration replicate certain mailboxes to both online and on-premises in order to enable fail over function?

2. For mailboxes that are located on exchange online, when on-premises server is not available, can they be used to send and receive mail then?

3. Are the O365 licenses needed only for users whose mailboxes are located on exchange online or for both on-premise and online in order to use their mailboxes in hybrid configuration?

The final approach is to enable us to use both online and on-premise mailboxes, so the online mailboxes be more resistant on on-premise infrastructure failures.

[#2]

Here's my take on your questions @Marko Farkas:

1. No, the default Exchange hybrid is not a backup/HA configuration so you can't have a copies of the mailbox both on-premises and in EXO. There may be third party tools that do that, but I haven't heard of any.

2. As long as you don't have centralised mail flow you will be able to send and receive mail from mailboxes located in EXO if the on-premises Exchange server is offline

3. My understanding is that you don't need a license for the mailboxes located on-premises, but they won't have access to any of the other services available (Office 365 ProPlus, SharePoint Online, OneDrive, etc) unless they are licensed

[#3]

@Marko Farkas Hey There,

Here is the answer for the questions put up by you - 

1. Can hybrid configuration replicate certain mailboxes to both online and on-premises in order to enable fail over function? -

No, and EXO or On-Premise is in no way designed to do a Backup for HA solutions to each other. They both are separate infrastructures (hosting) and you can never have a Mailbox with same email address/smtp in both forests when you have a Hybrid Deployment already ( AAD Sync replicating user info to Azure AD :) ) In exchange online Microsoft does the Maintenance work for you related to Infrastructure and they have 99.99 % SLA Dedicated to availability backed by Financial Refund, just in case if there is any Failure in above contract, so never fear about any Data Loss on Azure/Office 365 end.

2. For mailboxes that are located on exchange online, when on-premises server is not available, can they be used to send and receive mail then? - If your MX is pointing to Online and whether or not you have Mailboxes on-premise or not or Centralized Mail flow or not, in case of failures, emails wouldn't be delivered to On-Premise mailboxes/recipients. If an email is not delivered or relayed over to on-premise server, in no way you can back them up in Online Mailboxes. There are exceptions to it, by a functionality called transport rule ( Condition if Email is using Outbound Connector to my On-premise server, action - bcc or save a copy on this mailbox which is online - Workaround only and Microsoft never supports this scenario)

3. Are the O365 licenses needed only for users whose mailboxes are located on exchange online or for both on-premise and online in order to use their mailboxes in hybrid configuration? - You would need licenses for User Mailboxes only and not others like Shared/Room/Resource. If you happen to have Mailboxes over 50 GB in size for Shared/Room/Resource, you would need license in that case only, or if you need archiving solution for these, then only License is required for mailboxes other than User Mailboxes.

Hope that answers all your questions :) 

Cheers !

Ankit Shukla