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SMTP Shared Namespace and Distribution Groups

We have the short term need to setup an SMTP Shared Namespace on a single email domain.
At the moment the primary MX is setup to send mail to O365 first.  Then O365 has an Internal Relay setup to point to an Exchange 2007 for any addresses not local to O365.  Obviously the Exchange 2007 box has the same internal relay setup to cover internal mails for users based back on O365.
Mails addressed to specific people works fine between servers as does incoming mail from external users to the new domain.

My question is how to cover a distribution group that is split across sites.
Rather than adding 100 odd mail contacts at each site, we figured the best way would be, for each cross site list, to have a group at each site with two SMTP addresses, one for shared domain.com and one for nonshareddomain.com.
I then figured if we setup mail contacts at the opposite site to have a contact for group@nonshareddomain.com it would cover it i.e

everyone@shareddomain.com contains a list of local users to the local mail server.
On O365, that list also includes a contact for everyonerelay@nonshareddomain.com

That way, if an O365 user sends a message to everyone@, that message gets relayed to the other site, which in turn distributes it to all the other sites local users.

However, am I right in thinking that if we have a smiler relay contact setup in everyone@shareddomain in the other site, we will potentially create an infinite loop, as it will be trying to send back to everyonerelay@nonsharedomain.com at the first site.

Hope this ramble makes sense.
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Neil Potter
Asked:
Neil Potter
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1 Solution
 
IvanSystem EngineerCommented:
Hi,

regarding that internal relay setup..if you don't have server that is authorative for a domain, and some one send an email to a person that does not exist..what is going to happened to that email?
Endless loop between Office 365 and Exchange 2007..

PS: I just read last part of you message...yeah, infinitive loop.
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Neil PotterIT ManagerAuthor Commented:
Yeah thats what i'm thinking.  Is there any way around it?

With it being O365 Exchange at one end and 2007 at the other, in theory they have the default 30 hop limit anyway, but will this just bounce back and forth 30 times then stop, or will it actually deliver up to 30 emails to the same people?
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IvanSystem EngineerCommented:
Correct setup, if you don't have hybrid deployment would be to setup Exchange 2007 as authorative, and make it host all mailboxes.
Then you configure forwarding, so that when email for users on Office 365 is sent from on-premiss users, it is forwarded to their username@domain.onmicrosoft.com email.

Email does get received to Exchange 2007 mailbox, so you have to make sure mailbox is not full,
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IvanSystem EngineerCommented:
When on-premiss user want to send email to Office 365 user:

on-premiss user --> Exchange 2007 (user@domain.com) --> server forward rule to user@domain.onmicrosoft.com
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IvanSystem EngineerCommented:
It would bounce, would not get delivered many times.
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Gareth GudgerCommented:
Can you explain the short term need a little more? Just curious what the business need was behind splitting your namespace.

What about taking on premise to Exchange 2013 and configuring Hybrid?
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Neil PotterIT ManagerAuthor Commented:
We are going through a company merger.  My company has O365 with DirSync.  The other company has on premise Exchange 2007.
We have used shared namespace for the new email domain.
So mail comes into our O365 first.  Any "unknown" users get passed on to their On Premise server to be handled or bounced.
On their server, if one of their users sends a message to the new domain, if unknown it bounces it to O365.

We've had to take this approach as we have some data compliance issues preventing us from linking the sites and AD servers.  The long term plan is to link the sites, migrate the old AD's into a new single forest, and Stage Migrate their mailboxes up to O365.
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