An option would be, is to make 'free' email accounts, such as gmail , yahoo, ect, then you can send emails from a registered email account with (php) imap ,
your user's email server in this case doesn't block your mails.
i've made a little code to make direction depending on the email addresses,
lets try here: imap mail generator (beta)
But you need to create a few mail accounts.
At the end of the month, means you can use cron to run a server side application to send these mails. Most of the hosts allows you to add cron events on admin cpanels.
jjozsi.