MailHeaderAnalyzer

0.1.0

Private/Get-ExchangeClassification.ps1

                                # Exchange Online / hybrid classification headers (X-MS-Exchange-Organization-*,

# X-MS-Exchange-CrossTenant-*). Facts per the Exchange Team Blog article

# "Demystifying hybrid mail flow". Only AuthMechanism 10 is documented publicly.

function Get-ExchangeClassification {

    [CmdletBinding()]

    [OutputType([pscustomobject])]

    param([Parameter(Mandatory)][AllowEmptyCollection()][object[]]$Fields)

    $value = { param($name) Get-HeaderValue -Fields $Fields -Name $name }

    $directionality = & $value 'X-MS-Exchange-Organization-MessageDirectionality'

    $authAs = & $value 'X-MS-Exchange-Organization-AuthAs'

    $authSource = & $value 'X-MS-Exchange-Organization-AuthSource'

    $authMechanism = & $value 'X-MS-Exchange-Organization-AuthMechanism'

    $originatorOrg = & $value 'X-OriginatorOrg'

    $ctAuthAs = & $value 'X-MS-Exchange-CrossTenant-AuthAs'

    $ctAuthSource = & $value 'X-MS-Exchange-CrossTenant-AuthSource'

    $ctId = & $value 'X-MS-Exchange-CrossTenant-Id'

    $ctFrom = & $value 'X-MS-Exchange-CrossTenant-FromEntityHeader'

    $wrongTenant = & $value 'X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp'

    $preserved = & $value 'X-OrganizationHeadersPreserved'

    $filtered = & $value 'X-CrossPremisesHeadersFilteredBySendConnector'

    $anyOrganization = @($Fields | Where-Object { $_.Name -like 'X-MS-Exchange-Organization-*' }).Count -gt 0

    if (-not $anyOrganization -and -not $originatorOrg -and -not $ctAuthAs -and -not $ctId -and -not $ctFrom) { return $null }

    $directionalityMeaning = $null

    switch -Regex ($directionality) {

        '^Originating$' { $directionalityMeaning = 'Exchange Online classifies the message as coming from the organization itself (inbound connector of type OnPremises matched, or the sender is an Exchange Online mailbox).' }

        '^Incoming$' { $directionalityMeaning = 'Exchange Online classifies the message as coming from outside: delivered to an accepted domain without matching an inbound connector of type OnPremises.' }

    }

    $authAsMeaning = $null

    switch -Regex ($authAs) {

        '^Internal$' { $authAsMeaning = 'Intra-organizational: EOP skips spam, spoof, phishing and impersonation checks for inbound mail.' }

        '^Anonymous$' { $authAsMeaning = 'External: spam filters apply, authentication-requiring distribution lists reject it, Office documents open in Protected View.' }

    }

    $authMechanismMeaning = $null

    if ($authMechanism) {

        if ($authMechanism -match '^0*10$') {

            $authMechanismMeaning = 'Receive connector with "externally secured" permissions: everything from that source is marked Internal and bypasses EOP filtering.'

        } else {

            $authMechanismMeaning = 'Mechanism code {0}; the remaining codes are not publicly documented by Microsoft.' -f $authMechanism

        }

    }

    $ctFromMeaning = $null

    switch -Regex ($ctFrom) {

        '^Internet$' { $ctFromMeaning = 'Submitted from outside Office 365.' }

        '^Hosted$' { $ctFromMeaning = 'From an Office 365 tenant (mailbox in Exchange Online).' }

        '^HybridOnPrem$' { $ctFromMeaning = "From the organization's own on-premises Exchange via the hybrid connector." }

    }

    [pscustomobject]@{

        PSTypeName                 = 'MailHeaderAnalyzer.ExchangeClassification'

        Directionality             = $directionality

        DirectionalityMeaning      = $directionalityMeaning

        AuthAs                     = $authAs

        AuthAsMeaning              = $authAsMeaning

        AuthSource                 = $authSource

        AuthMechanism              = $authMechanism

        AuthMechanismMeaning       = $authMechanismMeaning

        OriginatorOrg              = $originatorOrg

        CrossTenantAuthAs          = $ctAuthAs

        CrossTenantAuthSource      = $ctAuthSource

        CrossTenantId              = $ctId

        CrossTenantFromEntity      = $ctFrom

        CrossTenantFromMeaning     = $ctFromMeaning

        WrongTenantAttribution     = $wrongTenant

        OrganizationHeadersPreserved = $preserved

        CrossPremisesHeadersFiltered = $filtered

    }

}