How to Send bulk mails using javax.mail API efficiently? & Can we use reuse authenticated sessions to improve speed?

I am able to send a mail using javax.mail API. But the problem here is on an average for each mail it taking around 4.3 seconds to send to destination.

If I am sending a 20 mails sequentially, it takes around 86.599 seconds. For my requirement this approach will not work. I am looking for an approach which can send large number of mails in less time.

When I looked at the debug log, the API is trying to authenticate to SMTP server for each and every message it sending. But I am creating a session only one time and using the same session for all the mails I am sending. Now my question is Isn’t it a overhead process every time authenticating itself to the smtp server. Isn’t there a better approach ?

Below is the log trace you may find helpful.

250-AUTH LOGIN PLAIN XOAUTH XOAUTH2
250 ENHANCEDSTATUSCODES
DEBUG SMTP: Found extension "SIZE", arg "35882577"
DEBUG SMTP: Found extension "8BITMIME", arg ""
DEBUG SMTP: Found extension "AUTH", arg "LOGIN PLAIN XOAUTH XOAUTH2"
DEBUG SMTP: Found extension "ENHANCEDSTATUSCODES", arg ""
DEBUG SMTP: Attempt to authenticate
DEBUG SMTP: check mechanisms: LOGIN PLAIN DIGEST-MD5 NTLM 
DEBUG SMTP: AUTH LOGIN command trace suppressed
DEBUG SMTP: AUTH LOGIN succeeded

Please let me know your thoughts on this and any help on this is really appreciated.

-Narendra

Answer

How are you sending the messages? The JavaMail FAQ suggests that the static Transport.send method will open a fresh connection for each message, as it is a convenience method that creates a suitable Transport instance, connects it, calls sendMessage and then closes the connection again. If you get your own Transport instance from the Session you can connect once, then call sendMessage repeatedly to send several messages on the one connection, and finally close it. Something along the lines of (untested):

Transport t = session.getTransport();
t.connect();
try {
  for(Message m : messages) {
    m.saveChanges();
    t.sendMessage(m, m.getAllRecipients());
  }
} finally {
  t.close();
}

Updated to use try with resources block:

try (Transport t = session.getTransport()) {
    t.connect();
    for(Message m : messages) {
        m.saveChanges();
        t.sendMessage(m, m.getAllRecipients());
    }
}