#Hash tables to store results of previous MX checks
$positiveMX = @{}
$negativeMX = @{}

#Adding this just so you have some visual for how far along you are.  It will slow it down slightly, so if you're
#purely worried about speed (or not running it interactively), remove this and the 4 $count lines in the loop
$count = 0

#Prep the CSV with headers
"domain,address" | set-content c:\temp\log.txt

#Read the file.  Using pipelining to avoid storing the entire thing in memory
get-content "C:\temp\email.txt" | foreach {
    $count++
    if ($count%500 -eq 0) {
        write-host $count -foregroundcolor black -backgroundcolor white
    }


    $domain= $_.split("@")[1]

    #If the MX record passed the test previously, go ahead and pass it through for writing
    if ($positiveMX.ContainsKey($domain)) {
        "$domain,$_"
    #Else, check to see if we found it before but disliked it, and if so do nothing
    } elseif ($negativeMX.ContainsKey($domain)) {
    #Else, haven't seen it before, need to do an MX Lookup
    } else {
        #Note that I changed this from '*yahoo.com' to '*yahoo*', since I tried using yahoo.com to test, and that ended with yahoodns.net.
        #If you know for sure that you want *yahoo.com, change it back
        $mx = nslookup -type=mx $domain | where {$_ -like '*yahoo*'}

        #Found it, adding it to the positive list and passing through for writing
        if($mx){
            "$domain,$_"
            $positiveMX.Add($domain,"")
        #Didn't find it, adding it to the bad list
        } else {
            $negativeMX.Add($domain,"")
        }
    }
} | add-content c:\temp\log.txt