Quick and dirty fix for AD/0365 users who are sending from their onmicrosoft account

FixProxyAddresses.ps1

Get-ADUser -Filter * `
           -SearchBase (Get-ADOrganizationalUnit -Filter 'Name -like "*MY OU*"') `
           -Properties ProxyAddresses,Mail |
ForEach-Object {

    if (!($_.Mail)) {
        Write-Host ($_.SamAccountName + ' - No mail attribute defined') -ForegroundColor Red
    } else {
        $NewProxyAddress = ('SMTP:' + $_.Mail)

        if ($_.ProxyAddresses -notcontains $NewProxyAddress) {
            Write-Host ($_.SamAccountName + ' - adding entry for ' + $NewProxyAddress) -ForegroundColor green
            $_.ProxyAddresses.Add($NewProxyAddress) | Out-Null            
            Set-ADUser -Instance $_
        } else {
            Write-Host ($_.SamAccountName + ' - already has entry for ' + $NewProxyAddress) -ForegroundColor Yellow
        }
    }
}