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Mailflow between Exchange 365 and on premise Exchange 2007 after Staged Migration


I just started with Office 365 and would like to migrate 40 users from Exchange 2007 using Staged Migration. I’ve done the research and feel that understand the process reasonably well. I understand that during the migration the mail flow between on premise exchange and Exchange Online is done using TargetAddress attributes and once all users are migrated I need to convert on premises mailboxes to Mailbox Enabled Users and point MX record to Exchange Online.
However, when the migration is done, I don’t want to decommission Exchange 2007 as need to keep some service mailboxes on Exchange 2007 and ensure there is a mailflow between Exchange Online and on premise one. I other words I want to achieve that if an email is sent to on premise mailbox, Exchange online would redirected to Exchange 2007. Emails sent from Exchange 2007 should go out straight to internet. Also I need successful mailflow between mailboxes on Exchange Online and Exchange 2007.
So my question is whether I can set up connectors (or something else) between on premise Exchange 2007 and Exchange online so mail flow is achieve between all mailboxes on both Exchanges.

I understand that using Hybrid migration would do the job but it involves installing Exchange 2013 and set up is more complex. Also, we don’t need rich co-existence so Hybrid is just an overkill for us.
Any help would be much appreciated.
Milan Duchovny
Milan Duchovny
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1 Solution
Vasil Michev (MVP)Commented:
Keep the MX pointing to on-prem and configure the domain as "Internal relay" in EO (O365 admin portal -> EAC -> Mail Flow -> Accepted domains -> double-click the domain -> select Internal relay). Once you do this, EO will redirect any messages for non-existing recipients associated with this domain to where your MX points at.
Milan DuchovnyAuthor Commented:
Thanks for the comment Vasil.

I wanted to to have MX record pointing at EO as it does spam filtering etc. and it would remove dependency on on-prem Exchange. But if that's only way I could live with that I guess.
If I implemented your suggestion, I'm assuming that emails sent from on-prem mailboxes would reach EO users using TargetAddress attributes even after I deleted staged migration batch. Is that correct?
Vasil Michev (MVP)Commented:
Well, if you want to keep the MX pointing to EO, you can simply use a subdomain for all the on-prem mailboxes. Otherwise you need some form of forwarding (regular forwarding, targetaddress based one, connectors, etc).
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Milan DuchovnyAuthor Commented:
Hi Vasil
Unfortunately the subdomain isn't an option for us.
Do you think that setting up a connector between on-prem and EO would work when using staged migration? Theoretically I don't see a reason why not but it seems that it shouldn't be set up with staged migration.

If had to, I could live with MX record pointing to on-prem and configure the domain as "Internal relay" but can you answer the question in my previews comment - "If I implemented your suggestion, I'm assuming that emails sent from on-prem mailboxes would reach EO users using TargetAddress attributes even after I deleted staged migration batch. Is that correct?"
Vasil Michev (MVP)Commented:
You need to set the domain to internal relay either way. Connector will work, and is the only solution that matches all your requirements. Just make sure that you dont have matching mailbox objects in the cloud.

I was proposing the simplest solution which doesnt require any connector, but it does require you to keep the MX pointing on-prem in order to avoid mail loops. Just to confirm, mail delivery will work just fine based on the targetaddress value even after you delete the batch.
Milan DuchovnyAuthor Commented:
Thanks Vasil
I'll go for the connectors set up as that's what I wanted to do from the beginning but when I spoke with Microsoft "engineer" he said it's not possible. I found it hard to believe, thus my question here :-)
Anyway the Microsoft engineer got back to me just now with below response confirming that connectors would work but are "not recommended".

Based on our last phone conversation, we understand that you would like to migrate 40 users to Exchange online using ‘Stage migration’ and want to keep few users in Exchange server 2007. This method is not recommended by Microsoft. However, you may try below mentioned steps to achieve this.
1.      Once you complete migration of all 40 users, you need to change the On Prem mailboxes to mail enabled users and then point your MX record to Office 365 Exchange online.
2.      Set Outbound connector in office 365 to route emails to On Prem Exchange server(Smart host is recommended to route emails to On prem)
3.      Set receive connector in EMC to receive messages from office 365. Add domain.onmicrosoft.com in address space to avoid looping. Kindly refer the link https://technet.microsoft.com/en-us/library/dn163583(v=exchg.150).aspx to add Office 365 IP’s in exchange on prem.
4.      Create send connector In EMC to send emails to office 365 and create inbound connector in office 365 to accept messages from On prem server.
5.      Default connector in On prem sends email directly to internet so no changes required.
6.      It is very important to close the firewall and just allow office 365 IP’s in allow list.
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