function Invoke-MtaSts
{
    [CmdletBinding()]
    param(
        [Parameter(
            Mandatory = $True,
            ValueFromPipeline = $True,
            ValueFromPipelineByPropertyName = $True,
            HelpMessage = "Specifies the domain for resolving the MTA-STS record."
        )][string]$Name
    )

    # Test if the MX record mail server supports TLS
    #Test-MxTls -MxHostname aspmx.l.google.com
    function Test-MxTls
    {
        [CmdletBinding()]
        param(
            [string]$MxHostname
        )

        # https://www.checktls.com/TestReceiver
        # https://www.alitajran.com/test-smtp-connection-with-telnet-powershell-script/

        try
        {
            $ret = @()
            $socket = New-Object System.Net.Sockets.TcpClient($MxHostname, 25)

            $stream = $socket.GetStream()
            $writer = New-Object System.IO.StreamWriter($stream)
            $buffer = New-Object System.Byte[] 1024
            $encoding = New-Object System.Text.AsciiEncoding
            Start-Sleep -Milliseconds 100
            While ( $stream.DataAvailable )
            {
                $read = $stream.Read($buffer, 0, 1024)
                $ret += $encoding.GetString($buffer, 0, $read)
            }

            $writer.WriteLine("EHLO TestingTLS")
            $writer.Flush()
            Start-Sleep -Milliseconds 100
            While ( $stream.DataAvailable )
            {
                $read = $stream.Read($buffer, 0, 1024)
                $ret += $encoding.GetString($buffer, 0, $read)
            }

            $writer.WriteLine("STARTTLS")
            $writer.Flush()
            Start-Sleep -Milliseconds 500
            While ( $stream.DataAvailable )
            {
                $read = $stream.Read($buffer, 0, 1024)
                $ret += $encoding.GetString($buffer, 0, $read)
            }

            return !!($ret -match "220")
        }
        catch
        {
            Write-Error $_
            return $false
        }
    }

    # check if the MX records are all covered by the MTA file and there's no extra or few MTA MX records configured. return true if all ok
    function Check-MxMta
    {
        [CmdletBinding()]
        param(
            [string]$domainName,
            [string]$mtsStsFileContents
        )

        $_mx = Resolve-DnsName -Name $domainName -Type MX | Select-Object -ExpandProperty NameExchange
        Write-Verbose "MX: $($_mx)"
        $_mta = $mtsStsFileContents.Split("`n") -match '(?<=mx: ).*$' | ForEach-Object { $_.Replace("mx:", "").Trim() }
        Write-Verbose "MTA: $($_mta)"

        # get all MX and MTA matches
        $ret = $_mx | ForEach-Object { $i = $_; $_mta | ForEach-Object { if ( $i -like $_ ) { [PSCustomObject]@{ MX = $i; MTA = $_} } } }
        Write-Verbose "Matches: `n $($ret | Out-String)"

        $res = Compare-Object -ReferenceObject $ret.MX -DifferenceObject (Resolve-DnsName -Name $domainName -Type MX | Select-Object -ExpandProperty NameExchange) # if differences, some MX records are not in MTA
        $res += Compare-Object -ReferenceObject ($ret.MTA | Select-Object -Unique) -DifferenceObject $_mta

        return (!$res)
    }

    $cti = (Get-Culture).TextInfo

    $mtsStsDns = (Resolve-DnsName -Name "_mta-sts.$($Name)" -Type TXT -ErrorAction SilentlyContinue | Where-Object { $_.Name -eq "_mta-sts.$($Name)" -and $_.Strings -match "v=STSv1" } | Select-Object -ExpandProperty Strings) -join "`n"
    Write-Verbose "mtsStsDns: $($mtsStsDns)"

    try { $mtsStsFile = Invoke-WebRequest "https://mta-sts.$($Name)/.well-known/mta-sts.txt" -UseBasicParsing -DisableKeepAlive | Select-Object -ExpandProperty Content } catch { $mtsStsFile = $null }
    Write-Verbose "mtsStsFile: $($mtsStsFile)"

    $mtaRecord = $mtsStsDns

    switch -Regex ($mtsStsDns)
    {
        { !$_ } { $mtaAdvisory = "The MTA-STS DNS record doesn't exist. "; continue }
        { $_.Split("`n").Count -ne 1 } { $mtaAdvisory = "There are multiple MTA-STS DNS records. " }
        { $_ -notmatch 'STSv1' } { $mtaAdvisory = "The MTA-STS version is not configured properly. Only STSv1 is supported. " }
        { $_ -notmatch 'id=([^;\s]{1,32})(?:;|$)' } { $mtaAdvisory = "The MTA-STS id must be alphanumeric and no longer than 32 characters. " }
        default {

            switch -Regex ($mtsStsFile)
            {
                { !$_ } { $mtaAdvisory = "The MTA-STS file doesn't exist. "; continue }
                { $_ -notmatch 'version:\s*(STSv1)' } { $mtaAdvisory = "The MTA-STS version is not configured in the file. The only options is STSv1. " }
                { $_ -notmatch 'mode:\s*(enforce|none|testing)' } { $mtaAdvisory = "The MTA-STS mode is not configured in the file. Options are Enforce, Testing and None. " }
                { $_ -match 'mode:\s*(enforce|none|testing)' -and $_ -notmatch 'mode:\s*Enforce' } { $mtaAdvisory = "The MTA-STS file is configured in $($null = $_ -match 'mode:\s*(enforce|none|testing)'; $cti.ToTitleCase($Matches[1].ToLower()) ) mode and not protecting interception or tampering. " }
                { !($_.Split("`n") -match '(?<=mx: ).*$') } { $mtaAdvisory = "The MTA-STS file doesn't have any MX record configured. " }
                { $_.Split("`n") -match '(?<=mx: ).*$' -and ( !(Check-MxMta -domainName $Name -mtsStsFileContents $mtsStsFile) ) } { $mtaAdvisory = "The MTA-STS file MX records don't match with the MX records configured in the domain. " }


                { $_.Split("`n") -match '(?<=mx: ).*$' -and ( Resolve-DnsName -Name $Name -Type MX | Select-Object -ExpandProperty NameExchange | ForEach-Object { Test-MxTls -MxHostname $_ -Verbose } ) -contains $false }   { $mtaAdvisory = "At least one of the MX records configured in the MTA-STS file MX records list doesn't support TLS. " }
                { $_ -notmatch 'max_age:\s*(604800|31557600)'} { $mtaAdvisory = "The MTA-STS max age configured in the file should be greater than 604800 seconds and less than 31557600 seconds. " }
                default {
                    $mtaAdvisory = "The domain has the MTA-STS DNS record and file configured and protected against interception or tampering."
                }
            }
        }
    }

    return [PSCustomObject]@{ mtaAdvisory = $mtaAdvisory; mtaRecord = $mtaRecord}
}



Invoke-MtaSts -Name microsoft.com -Verbose

    $SPF = $SPF -join ""
    $spfCnt = ([regex]::Matches($SPF, "v=spf1" )).count
    if ($SPF -eq $null) {
        $SpfAdvisory = "Domain does not have an SPF record. To prevent abuse of this domain, please add an SPF record to it."
    }
    if($spfCnt -gt 1) {
        $SpfAdvisory = "Domain has more than one SPF-record. One SPF record for one domain. This is explicitly defined in RFC4408"
    }
    Else {
        switch -Regex ($SPF) {
            '~all' {
                $SpfAdvisory = "An SPF-record is configured but the policy is not sufficiently strict."
            }
            '-all' {
                $SpfAdvisory = "An SPF-record is configured and the policy is sufficiently strict."
            }
            "\?all" {
                $SpfAdvisory = "Your domain has a valid SPF record but your policy is not effective enough."
            }
            '\+all' {
                $SpfAdvisory = "Your domain has a valid SPF record but your policy is not effective enough."
            }
            Default {
                $SpfAdvisory = "No qualifier found. Your domain has a SPF record but your policy is not effective enough."
            }
        }
    }