input: Write a python function to send an email using smtplib.
  assert send_email('sender@example.com', 'receiver@example.com', 'Subject: Test\n\nThis is a test email.') == "Successfully sent email"
  assert send_email('invalid_sender@example.com', 'receiver@example.com', 'Subject: Test\n\nThis is a test email.') == "Error: unable to send email"
output: import smtplib
  from email.mime.text import MIMEText
  from email.mime.multipart import MIMEMultipart
  
  def send_email(sender, receiver, message):
      try:
          # SMTP server configuration
          smtp_server = "smtp.example.com"
          smtp_port = 587
          smtp_username = "your_username"
          smtp_password = "your_password"
          
          # Create the email
          msg = MIMEMultipart()
          msg['From'] = sender
          msg['To'] = receiver
          msg['Subject'] = message.split('\n')[0][9:]  # Extract subject
          
          msg.attach(MIMEText(message.split('\n\n', 1)[1], 'plain'))  # Extract body
          
          # Connect to the server and send email
          server = smtplib.SMTP(smtp_server, smtp_port)
          server.starttls()
          server.login(smtp_username, smtp_password)
          server.sendmail(sender, receiver, msg.as_string())
          server.quit()
          
          return "Successfully sent email"
      except Exception as e:
          return f"Error: unable to send email"