input: Write a function to classify an email as spam or not based on certain keywords.
  assert classify("Win a free prize now!") == "This is a spam email."
  assert classify("Hello, I hope you are doing well.") == "This is not a spam email"
  assert classify("Special offer just for you.") == "This is a spam email."
output: def classify(email):
      spam_keywords = ["win", "free", "prize", "now", "special offer"]
      email_lower = email.lower()
      for keyword in spam_keywords:
          if keyword in email_lower:
              return "This is a spam email."
      return "This is not a spam email."