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Mail getting delivered by second site

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we have a Exchange 2013 setup with Exchange 2010 which is due to be removed.
We have 2 AD sites with a Client Access server DRW-CAS / GSD-CAS  and mailbox server DRW-MB01 / GSD-MB01 in each site, the mailbox server`s are setup in a DAG with 2 mailboxes DB`s
When running a get-mailbox I see the below
Name                      Alias                ServerName
XXX               xxx              drw-mb01
Sadler                 Sadler               drw-mb01
Ram                   Ram                  drw-mb01
admin                admin           drw-mb01
xxxx            xxxx            gsd-mb01
xxxer                jcc              gsd-mb01
own                cc                gsd-mb01
Kean               Kcc              gsd-mb01
the first 4 users are in Mailbox DB1 and the other users are in Mailbox DB2, both mailboxes are mounted and active on gsd-mb01 so I am not sure why it shows they don't both show on GSD-MB01
The problem we see is Mail is Delivered via SMTP to GSD-CAS but then this email seems to route to the DRW-MB01 server and then back to the GSD-MB01 server, should the mail not go direct
to the GSD-MB01 server as (1) it`s in the same AD site (2) it`s got the active copy of the DB even though the get-mailbox command lists the other server, if we turn off the DRW-MB01 server mail does not flow
the headers in the email show the below
Received: from DRW-MB01 ( by
 gsd-mb01 ( with Microsoft SMTP Server (TLS) id
 15.0.847.32 via Mailbox Transport; Wed, 25 Jun 2014 12:59:49 +0100
Received: from gsd-cas ( by
 drw-mb01 ( with Microsoft SMTP Server (TLS) id
 15.0.847.32; Wed, 25 Jun 2014 12:59:49 +0100
Received: from mail6.bemta3.messagelabs.com ( by
 gsd-caht01 ( with Microsoft SMTP Server (TLS) id
 15.0.847.32 via Frontend Transport; Wed, 25 Jun 2014 12:59:48 +0100
Return-Path: <[email protected]>
Received: from [] by server-10.bemta-3.messagelabs.com id


How many recipients were there in the email?
If there are multiple recipients, CAS takes decision on sending it to a mailbox server which is in the closest Delivery Group. If there is single Recipient, it should pick the server with the closest Delivery Group and the Delivery Groups are
Routable DAG
Mailbox delivery group
AD site
Now, I am not sure of it is trying for a another site to keep a shadow copy there and hence achieve site resiliency for transport. Ideally it should have done a delivery to the Mailbox server in the same site and then made a copy of it on remote site server
and then get done with it if it is one recipient.
If multiple recipients and majority recipients fall in another site, then it picks the transport service on the mailbox server in another site and then that transport service takes decision on where to route the emails to.
Refer :

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