Recently I was migrating a customer from Exchange 2013 in Forest A to Exchange 2016 in Forest B. They wanted to keep the same shared namespace across both exchange environments.

Autodiscover and MX records was pointing to the source Exchange 2013. We used as a domain for mail routing to Exchange 2016 as follows:

User A has been migrated to Exchange 2016. User B is still on Exchange 2013. When userA was migrated then the External E-mailAddress was set as UserB looks up and finds a mail contact with an external email address of and is routed to exchange 2016 by DNS for If userA in Exchange 2016 tries to lookup userB then Autodiscover DNS is pointing to Exchange 2013 for If userA emails UserB then due to the domain in Exchange 2016 being set as an internal relay domain to delivers to exchange 2013 via a send connector created.

Forest A:



Mail Contact: , externalemailaddress


Forest B:



Mail Enabled User:


For mail flow and free/busy information to work the following steps must be performed. This is assuming that the users have been migrated by ADMT or another means.

  • In the source domain setup DNS for to point to Exchange 2016 server(s).
  • In the target environment (forest B) export the autodiscoverconfig from forest A.

o   Export-AutodiscoverConfig -TargetForestDomainController “dc.foresta.local” -TargetForestCredential (Get-Credential) -MultipleExchangeDeployments $true

  • Setup ADPermissions on within each forest.

o   Get-MailboxServer | Add-ADPermission -Accessrights Extendedright -Extendedrights “ms-Exch-EPI-Token-Serialization” -User “<Remote Forest Domain>\Exchange servers”

  • Add AvailabilityAddressSpace – Forestname is smtp domain being used.

o   In Forest A:

  • Add-AvailabilityAddressSpace -Forestname -AccessMethod PerUserFB -UseServiceAccount:$true

o   In Forest B:

  • Add-AvailabilityAddressSpace -Forestname -AccessMethod PerUserFB -UseServiceAccount:$true
  • Enable-MailUser in the target forest (forest B in this case). Without the user mail enabled the Free/Busy will fail.

Prepare-moverequest for the user in the exchange management shell of target forest (forest B). MRSProxy needs to be enabled to migrate mailboxes.

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