blocklist = ('disposable_email_blocklist.conf')
blocklist_content = [line.rstrip() for line in blocklist.readlines()]
if email.split('@')[1] in blocklist_content:
    message = "Please enter your permanent email address."
    return (False, message)
else:
    return True

>>> from disposable_email_domains import blocklist
>>> 'bearsarefuzzy.com' in blocklist
True

function isDisposableEmail($email, $blocklist_path = null) {
    if (!$blocklist_path) $blocklist_path = __DIR__ . '/disposable_email_blocklist.conf';
    $disposable_domains = file($blocklist_path, FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
    $domain = mb_strtolower(explode('@', trim($email))[1]);
    return in_array($domain, $disposable_domains);
}

before_validation :reject_email_blocklist

def reject_email_blocklist
  blocklist = File.read('config/disposable_email_blocklist.conf').split("\n")

  if blocklist.include?(email.split('@')[1])
    errors[:email] << 'invalid email'
    return false
  else
    return true
  end
end

'use strict';

const readline = require('readline'),
  fs = require('fs');

const input = fs.createReadStream('./disposable_email_blocklist.conf'),
  output = [],
  rl = readline.createInterface({input});

// PROCESS LINES
rl.on('line', (line) => {
  console.log(`Processing line ${output.length}`);
  output.push(line);
});

// SAVE AS JSON
rl.on('close', () => {
  try {
    const json = JSON.stringify(output);
    fs.writeFile('disposable_email_blocklist.json', json, () => console.log('--- FINISHED ---'));
  } catch (e) {
    console.log(e);
  }
});

private static readonly Lazy<HashSet<string>> _emailBlockList = new Lazy<HashSet<string>>(() =>
{
  var lines = File.ReadLines("disposable_email_blocklist.conf")
    .Where(line => !string.IsNullOrWhiteSpace(line) && !line.TrimStart().StartsWith("//"));
  return new HashSet<string>(lines, StringComparer.OrdinalIgnoreCase);
});

private static bool IsBlocklisted(string domain) => _emailBlockList.Value.Contains(domain);

...

var addr = new MailAddress(email);
if (IsBlocklisted(addr.Host)))
  throw new ApplicationException("Email is blocklisted.");