If you have ever hosted a reasonably good site on a shared hosting service then you would definitely have come across the dreaded "exceeded the email limit per hour" warning. If you use Drupal then you don't have to worry about this any more. If you are using Drupal then all you have to do is to install the MailQ module to queue all your mails.
Once the mails are queued, they will be processed later during cron runs where the queue will be emptied in batches of size set by the MailQ module. You can set the batch size based on the frequency of cron runs and the limit on the number of emails per hour allowed by your hosting provider. Normally, shared hosting providers also sets limits on the frequency of cron jobs as well. Whatever be the limits you have the system fully under your control and you do not have to worry about overshooting your email limits if you use the MailQ Module