Office 365: Add Anti-Spoofing Protection Transport Rule with PowerShell

This will send to quarantine e-mails from *@domain.com to *@domain.com which came from outside the organization and failed SPF.

# https://admin.exchange.microsoft.com/#/transportrules

Connect-ExchangeOnline

$AcceptedDomains = Get-AcceptedDomain

foreach ($DomainEntry in $AcceptedDomains) {
    $DomainName = $DomainEntry.DomainName
    
    # Skip the default Microsoft domains
    if ($DomainName -like "*.onmicrosoft.com") {
        continue
    }

    $RuleName = "Anti-Spoofing Protection - $DomainName"

    # Check if a rule with this name already exists
    if (Get-TransportRule -Identity $RuleName -ErrorAction SilentlyContinue) {
        Write-Host "Rule already exists for $DomainName. Skipping..." -ForegroundColor Yellow
    }
    else {
        Write-Host "Creating rule for $DomainName..." -ForegroundColor Cyan
        New-TransportRule -Name $RuleName `
            -SenderDomainIs $DomainName `
            -FromScope "NotInOrganization" `
            -Quarantine $true `
            -ExceptIfHeaderContainsMessageHeader "Authentication-Results" `
            -ExceptIfHeaderContainsWords "spf=pass"
    }
}