PEAR::Mail, how to send newsletters?

Hi, I want to send a newsletter to 4000 recipients but my server says me: SMTP: Invalid response code received from server (code: 421, response: 4.3.2 Too many open connections.) Then I think I'm not doing a god job. I use Mail ans Mail_mime to send the newsletter that is in html format. I have tried it making one instance of the class for each recipient and sending one mail for each recipient. But I also have tried to chunk my recipients array and send emails to 50 recipients in each loop. I use the smtp option with autentication and the persists option set to true. There is a way to do this job more efficiently ? I don't know if I get this error from server from previeous tests this afternoon. Tomorrow I'll do more tests. I would apreciate some orientation. Thanks. Leo