Sending out mass emails

Hi all, We would like to send out mass emails for some of our clients, these are HTML email and text alternative for the email clients that cant read those HTML emails. We have developed some scripts to send the emails using phpmailer. Now we anticipate sending emails in batches but not sure how many at once. Also what is the best way about going around being black listed due to spam issues. I know that the headers need to be set to avoid being detected as spam. Any advice is appreciated. Thanks Angelo