Connect-ExchangeOnline
# Define the domain to remove
$domainToRemove = "domain.com.au"

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

foreach ($mailbox in $mailboxes) {
    # Get current email addresses
    $currentAddresses = $mailbox.EmailAddresses

    # Filter out addresses with the specified domain
    $updatedAddresses = $currentAddresses | Where-Object { $_ -notlike "*@$domainToRemove" }

    # Update the mailbox if any addresses were removed
    if ($currentAddresses.Count -ne $updatedAddresses.Count) {
        Write-Host "Updating mailbox for: $($mailbox.PrimarySmtpAddress)"
#Fixes WARNING: Proxy address  is used as WindowsLiveId. So it can't be removed from list of email addresses. To remove it, first change
the WindowsLiveId.
		Set-Mailbox -Identity $mailbox.PrimarySmtpAddress -MicrosoftOnlineServicesID $mailbox.PrimarySmtpAddress
        Set-Mailbox -Identity $mailbox.Identity -EmailAddresses $updatedAddresses

    } else {
       # Write-Host "No changes needed for: $($mailbox.PrimarySmtpAddress)"
    }
}

Write-Host "Completed processing all users."