How invite and mass mailer plugins works?

I mean, let's say that a user invite all his friends, o the admin has to send a mail to all the users, and there'll we a lot of emails to be sent (could be 100, 500, 1000, 10000 or more), how these two plugins work? do they send all the email all by one shot (the best way to have the ip banned ) or do they send a chunk (how big?) of them every X minutes using a cron (or stuff like that)?