Migrate mailbox to another domain same Tenant

Hi gteibo1,
Change the UPN with Powershell:
# Define the new UPN suffix
$newUPNSuffix = "new-domain.com"

# Connect to Azure AD
Connect-AzureAD

# Get all users in Azure AD
$users = Get-AzureADUser -All $true

# Loop through each user and update their UPN
foreach ($user in $users) {
# Extract the username part from the current UPN
$username = ($user.UserPrincipalName).Split('@')[0]

# Construct the new UPN
$newUPN = "$username@$newUPNSuffix"

# Set the new UPN
Set-AzureADUser -ObjectId $user.ObjectId -UserPrincipalName $newUPN
}

Write-Host "UPNs have been updated for all users."

After hat change der primary SMTP address:
# Define the new primary SMTP domain
$newPrimarySMTPDomain = "newdomain.com"

# Get all mailboxes
$mailboxes = Get-Mailbox -ResultSize Unlimited

# Loop through each mailbox and update the primary SMTP address
foreach ($mailbox in $mailboxes) {
$currentPrimarySMTP = $mailbox.PrimarySmtpAddress
$newPrimarySMTP = $mailbox.UserPrincipalName.Replace('@', "@$newPrimarySMTPDomain")

# Set the new primary SMTP address
Set-Mailbox -Identity $mailbox.Identity -PrimarySmtpAddress $newPrimarySMTP -EmailAddressPolicyEnabled $false

Write-Host "Changed primary SMTP address for $($mailbox.DisplayName) from $currentPrimarySMTP to $newPrimarySMTP"
}

Write-Host "Primary SMTP addresses have been updated for all mailboxes."