When an internal Active Directory domain matches an external internet domain not owned by the organization, it creates a classic namespace collision scenario. In your case with Server 2008 R2 and Exchange 2010, Microsoft's official stance is clear: domain rename operations are unsupported in environments with Exchange 2010 due to the tight AD-Exchange integration.

For organizations with substantial infrastructure where forest migration would be prohibitively expensive, consider these alternative approaches:

1. UPN Suffix Configuration

# PowerShell: Adding alternative UPN suffixes
Get-ADForest | Set-ADForest -UPNSuffixes @{Add="newdomain.com"}
Get-ADUser -Filter * | ForEach-Object {
    $newUpn = $_.SamAccountName + "@newdomain.com"
    Set-ADUser $_ -UserPrincipalName $newUpn
}

2. SMTP Address Rewriting

Configure Exchange to accept emails for both domains while presenting the correct external domain:

# Exchange Management Shell
Get-Mailbox -ResultSize Unlimited | Set-Mailbox -EmailAddressPolicyEnabled $false
Get-Mailbox -ResultSize Unlimited | ForEach-Object {
    Set-Mailbox $_ -EmailAddresses @{Add=($_.PrimarySmtpAddress.ToString().Replace("@olddomain.com","@newdomain.com"))}
}

3. Split DNS Configuration

Implement a split-brain DNS solution to maintain internal resolution while using the proper external domain publicly:

; DNS Zone File Example
@    IN    A    192.168.1.1
mail IN    A    192.168.1.2
autodiscover IN CNAME mail

For applications using Kerberos/NTLM authentication with hardcoded domains:

# Registry modification for SPN handling
reg add "HKLM\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0" /v "BackConnectionHostNames" /t REG_MULTI_SZ /d "olddomain.com\0newdomain.com" /f

When dealing with Exchange certificates, ensure you include both names:

New-ExchangeCertificate -GenerateRequest -SubjectName "dc=com,dc=newdomain,cn=mail" -DomainName mail.newdomain.com,autodiscover.newdomain.com,mail.olddomain.com -PrivateKeyExportable $true

While these workarounds address immediate concerns, begin planning for:

• Exchange 2010 end-of-life (October 2020)
• Windows Server 2008 R2 end-of-support
• Gradual migration to a properly named AD forest

When inheriting an Active Directory (2008 R2 native) and Exchange 2010 environment, discovering that the internal domain matches an unowned external domain presents significant technical challenges. The core limitation stems from Microsoft's architectural design where Exchange 2010 tightly binds to the AD forest structure during installation.

Key factors preventing domain rename:

// Exchange 2010's dependency on AD attributes
Get-ADObject -Filter {objectClass -eq "msExchConfigurationContainer"} | 
Select-Object DistinguishedName,whenCreated

• Exchange 2010's configuration is stored in the Configuration partition
• Transport components hardcode original domain references
• CAS proxies embed original domain in authentication mechanisms

Option 1: Implement UPN suffix modification

# PowerShell to add UPN suffix
Set-ADForest -Identity current.forest -UPNSuffixes @{Add="newdomain.com"}
Get-ADUser -Filter * | ForEach-Object {
    $newUpn = $_.SamAccountName + "@newdomain.com"
    Set-ADUser $_ -UserPrincipalName $newUpn
}

Option 2: DNS namespace segregation

// DNS zone configuration example
Add-DnsServerPrimaryZone -Name "internal.corp" -ZoneFile "internal.corp.dns"
Set-ADDomain -Identity current.domain -ParentDomain internal.corp

When evaluating forest migration:

# Exchange 2010 cross-forest migration steps
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri http://oldExchange/PowerShell/
Import-PSSession $Session
Get-Mailbox -Server oldExchange | New-MoveRequest -TargetDatabase newExchangeDB

• Implement SMTP address policies to phase out old domain references
• Update SPF/DKIM records for email authentication
• Configure split-brain DNS for internal resolution