According to me, the first thing we need to deal with is CSV file. First let's read one CSV file, loop over it.

Here is an example of CSV input file.

input.csv

Samaccountname,Mail
abc123,abc123@domain.com
def123,def123@domain.com
ghi123,ghi123@test.com

csvLevel1.ps1

Import-Csv -Path input.csv | foreach {
 
    # save current object into different variable
    $currentLine = $_
 
    # deal with columns
    $mail = $currentLine.Mail
 
    # test domain mail
    # 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

This script will output CSV file with a new column.

preCheck.csv

"Samaccountname","Mail","domainCheck"
"abc123","abc123@domain.com","valid"
"def123","def123@domain.com","valid"
"ghi123","ghi123@test.com","invalid"