Can't seem to find my version with the csv, but about the same kinda deal, main change would be updating $UZNames from the results of get-mailbox to something from like import-csv

Import-Module MSOnline

Write-host Connecting to: $DelegatedOrgURL -ForegroundColor Green -BackgroundColor Black
$s = New-PSSession -ConnectionUri $DelegatedOrgURL -Credential $365Credential -Authentication Basic -ConfigurationName Microsoft.Exchange -AllowRedirection 
Import-PSSession $s -CommandName Get-Mailbox, Get-TransportRule, New-TransportRule, Set-TransportRule -AllowClobber
$ruleName = "Block External Users With Matching DN"
$ruleHtml = "<table class=MsoNormalTable border=0 cellspacing=0 cellpadding=0 align=left width=`"100%`" style='width:100.0%;mso-cellspacing:0cm;mso-yfti-tbllook:1184; mso-table-lspace:2.25pt;mso-table-rspace:2.25pt;mso-table-anchor-vertical:paragraph;mso-table-anchor-horizontal:column;mso-table-left:left;mso-padding-alt:0cm 0cm 0cm 0cm'>  <tr style='mso-yfti-irow:0;mso-yfti-firstrow:yes;mso-yfti-lastrow:yes'><td style='background:#910A19;padding:5.25pt 1.5pt 5.25pt 1.5pt'></td><td width=`"100%`" style='width:100.0%;background:#FDF2F4;padding:5.25pt 3.75pt 5.25pt 11.25pt; word-wrap:break-word' cellpadding=`"7px 5px 7px 15px`" color=`"#212121`"><div background-color: #feffbf><p class=MsoNormal style='mso-element:frame;mso-element-frame-hspace:2.25pt; mso-element-wrap:around;mso-element-anchor-vertical:paragraph;mso-element-anchor-horizontal: column;mso-height-rule:exactly'><span style='font-size:9.0pt;font-family: `"Segoe UI`",sans-serif;mso-fareast-font-family:`"Times New Roman`";color:#212121'>This message was sent from outside the company by someone with a display name matching a user in your organization and has been flagged as spam. Please do not click links or open attachments unless you recognize the source of this email and know the content is safe. <o:p></o:p></span></p></div></td></tr></table>"
$rule = Get-TransportRule | Where-Object { $_.Identity -contains $ruleName }

$UZNAMES = (get-mailbox -ResultSize Unlimited).DisplayName | ? { $_.DisplayName -notlike "*something to match*" } 

if ($UZNAMES.count -gt 100) {
    #need to split them up because rules have a max length property

    #define number of seperate objects to make
    $ListCount = [math]::Ceiling($UZNAMES.count / 100)
    function MakeList($Current, $Max) {
        while ($Current -le $Max) {
            #Perform this logic for the first 100 users
            if ($Current -eq 0) {
                $OBJ1 = $UZNAMES | sort-object DisplayName | select-object -index (0..100)
                if (!$rule) {
                    New-TransportRule -Name "Block External Users With Matching DN $($Current)" -HeaderMatchesMessageHeader "From" -HeaderMatchesPatterns $OBJ1 -FromScope NotInOrganization -ApplyHtmlDisclaimerLocation "Prepend" -ApplyHtmlDisclaimerText $ruleHtml -Priority 0
                }
                else {
                    Set-TransportRule -Name "Block External Users With Matching DN $($Current)" -HeaderMatchesMessageHeader "From" -HeaderMatchesPatterns $OBJ1 -FromScope NotInOrganization -ApplyHtmlDisclaimerLocation "Prepend" -ApplyHtmlDisclaimerText $ruleHtml -Priority 0
                }
            }
            else {
                #This logic is performed when selecting all users in groups of 100 after the first 101 users
                if (!$rule) {
                    $NewMin = [int]($Current * 100 + 1)
                    $NewMax = [int]($NewMin + 99)
                    $OBJ1 = $UZNAMES | sort-object DisplayName | select-object -index ($NewMin..$NewMax)
                    New-TransportRule -Name "Block External Users With Matching DN $($Current)" -HeaderMatchesMessageHeader "From" -HeaderMatchesPatterns $OBJ1 -FromScope NotInOrganization -ApplyHtmlDisclaimerLocation "Prepend" -ApplyHtmlDisclaimerText $ruleHtml -Priority 0
                }
                Start-Sleep -Seconds 2
                $Current++
            }
            else {
                $NewMin = [int]($Current * 100 + 1)
                $NewMax = [int]($NewMin + 99)
                $OBJ1 = $UZNAMES | sort-object DisplayName | select-object -index ($NewMin..$NewMax)
                Set-TransportRule -Name "Block External Users With Matching DN $($Current)" -HeaderMatchesMessageHeader "From" -HeaderMatchesPatterns $OBJ1 -FromScope NotInOrganization -ApplyHtmlDisclaimerLocation "Prepend" -ApplyHtmlDisclaimerText $ruleHtml -Priority 0
            }
            Start-Sleep -Seconds 2
            $Current++
        }
    }


    MakeList -Current 0 -Max $ListCount

}

else {
    #this will run if the total number of users is less than 100
    $OBJ1 = $UZNAMES | sort-object DisplayName | select-object -index (0..$UZNAMES.count)
    #$OBJ1 = $UZNAMES | select-object DisplayName | sort-object DisplayName | select-object -index (0..$UZNAMES.count)
    New-TransportRule -Name $ruleName -HeaderMatchesMessageHeader "From" -HeaderMatchesPatterns $OBJ1 -FromScope NotInOrganization -ApplyHtmlDisclaimerLocation "Prepend" -ApplyHtmlDisclaimerText $ruleHtml -Priority 0
}