input: Write a function to send emails to customers when their product is available.
  assert send_emails({'customer1@example.com': 'Product1', 'customer2@example.com': 'Product2'}, 'smtp.example.com', 587, 'username', 'password')==None
  assert send_emails({'customer3@example.com': 'Product3'}, 'smtp.example.com', 587, 'username', 'password')==None
output: import smtplib
  from email.mime.text import MIMEText
  
  def send_emails(customers, smtp_server, port, username, password):
      # Establish connection to the SMTP server
      server = smtplib.SMTP(smtp_server, port)
      server.starttls()
      server.login(username, password)
      
      # Iterate over each customer and send an email
      for email, product in customers.items():
          msg = MIMEText(f"Dear Customer,\n\nYour product {product} is now available.\n\nThank you!")
          msg['Subject'] = 'Product Availability Notification'
          msg['From'] = username
          msg['To'] = email
          
          server.sendmail(username, email, msg.as_string())
      
      # Quit the server connection
      server.quit()