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

csv.ps1

Import-Csv -Path input.csv | foreach {
 
  # save current object into different variable
  $currentLine = $_
 
  # deal with columns
  $sam = $currentLine.Samaccountname
 
  # let's see if the user exists into AD
  try {
    $user = Get-ADUser -identity $sam
    # user found
  } catch {
    # user not found, maybe user is disabled for instance ?
  }
 
}