Import-Csv -Path input.csv | foreach {

    # save current object into a local variable
    $currentLine = $_

    # deal with columns
    $mail = $currentLine.Mail

    # test mail domain name
    # we add a property with the result of our test
    if ($mail -match "domain.com$") {
        $currentLine | Add-Member -MemberType NoteProperty -Name "domainCheck" -Value "valid"
    } else {
        $currentLine | Add-Member -MemberType NoteProperty -Name "domainCheck" -Value "invalid"
    }
    # return updated currentLine with new column
    $currentLine
} | 
# export result line by line to CSV
# -NoTypeInformation prevents metadata from being exported
Export-Csv -Path preCheck.csv -NoTypeInformation