input: Write a function to send an email using smtplib.
  assert send_email('localhost', 'test@example.com', 'test@example.com', 'test', 'Hello World') == "Successfully sent email"
  assert send_email('localhost', 'invalid@example.com', 'test@example.com', 'test', 'Hello World') == "Error: unable to send email"
output: def send_email(server, from_addr, to_addr, subject, message):
      import smtplib
      from email.mime.text import MIMEText
  
      msg = MIMEText(message)
      msg['Subject'] = subject
      msg['From'] = from_addr
      msg['To'] = to_addr
  
      try:
          with smtplib.SMTP(server) as smtp:
              smtp.sendmail(from_addr, [to_addr], msg.as_string())
          return "Successfully sent email"
      except Exception as e:
          return "Error: unable to send email"