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Exchange 2013 resource forest - Outlook will not connect to the new Mail Server post-mailbox migration

Overview:
3 AD Domains, each hosting its own Microsoft Exchange environment. End game, we will most likey migrate all users/computers into the new domain, but Exchagne is the primary objective
Objective:
Migrate users to one Exchange System to ease administration burden
Plan:
Create new Resource Domain to host Exchange. Migrate users from the 3 existing Exchange footprints to the new one.

Two-Way forest trusts are in place and working across all 4 domains.
ADMT has been installed on a server in the New Targeted Exchagne Forest (for migrating users/computers)
PES Server has been installed in each Source Forest/Domain (not really needed at this stage)

The first step in the migration process, we run the PrepareMailboxMove.ps1 from the Newe Target Exchange Server, for the desired Source Exchagne User. This completes successfully, adding the x.500 proxy address variables to both Exchange Systems.

If we migrate a Mailbox to the new Exchagne System, everything migrates without a problem. The end-user is told "Outlook needs to be relaunched due to an administrator change" - once users close Outlook and relaunch it, Outlook will not connect to their mailbox. The issue seems to be Outlook not directing the client to the new Exchange Server, mail.newexchangesystem.com , instead, it continues trying to connect to mail.oldexchagnesystem.com and never attempts to look at the new Exchange System.
If we go into the Clients Outlook profile (on their computer) and tell it to "Connect to Microsoft Exchange using HTTP" and then specify the New Exchange URL in the "Exchange Proxy Server" address, the client does connect.

I had everyting working in this environment a short while ago, where, migrated Mailbox users had no problem with Outlook finding their *new* mailbox post-migration. At some point, something must've changed to cause the behavrior we are now running into.

Looking for some guidance. Thanks. 
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M A
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What Outlook version  did you try?

Did you try the same with Outlook 2016/2019?\

Did you try after enabling MAP-HTTP in Exchange?
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ASKER

Tried Outlook 2010 and 2013 - We will have quite a few Outlook 2010 clients using the system. The results did not seem to change between the two different versions of Outlook.

Resource Forest Exchagne, current Org. config shows that: MapiHttpEnalbed :False . I can enable this setting for the Resource foreset, but I do need to make sure that doesn't break anyhting for Outlook 2010 or the Exchange 2010 servers in the 3 existing AD Forests. I did enable MapiHttpEnabled:True on the resource forest Exchange system and retested with an Outlook 2013 client - did not change the behavior. 

To add: Once we set the "Exchange Proxy Settings" within Outlook (setting the Resource forest Exchange server URL as the Proxy Server to connect to)
We can get Outlook to connect. I did notice the following, however - If I try to "verify" the Mailbox User, pointing to the old Exchagne Server - it can't set the user up (server name = server.oldexchangesystem.com)
Once I set the "Server" variable to be server.newexchangesystem.com , it does "find" the user and can resolve the name back to the Source/User Forest - but, what occurs after the account is verified is the "Server" address changes from server.newexchangesystem.com to a string of numbers/letters@oldexchangesystem.com (see pic). 
 User generated image
Pretty let down by the "lack" of engagement within EE -
Def. doesn't seem to be worth the price of admission. 

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If you have the mailbox now in the resource domain you will have to create linked mailbox accounts for them to be able to access the mailbox in the resource domain. So your existing domain is classed now as the account domain.