title: When should I send QUIT to SMTP server? and how long should I keep a session?
body: <p>I am programming a smtp sender to send large number of distinct emails (not spam). As there are many destination addresses which are from common provider like hotmail.com/gmail.com. I would like to pool the TCP connections in order to reuse the session. Is this a good practise? Or should I disconnect and connect to send distinct mails instead?</p>
  
  <p>As the subscribers are in large amount, the pooled connection might be kept for a long time (e.g. 1 hour), should I actively release the connection at some point?</p>
  
  <p>Last thing confused me is the destination IPs, as I find out that hotmail.com has a lot of mx servers(at least 12). Does it make any different if I send most of emails to only one of the server or separate them and send to all servers?</p>
  
  <p>Sorry to ask several questions at a time, and do appreciate your view and answer.</p>