JMailQ

JMailQ captures all email passing through the Joomla system. It sends these emails via SMTP to an SMTP server. You can configure JMailQ to meter the emails out based on a number of criteria such as emails per minute, per hour & per day, as well as maximum cpu time spent on any one delivery process. You can also set a minimum duration between sending emails.

JMailQ also has 2 plugin interfaces with samples to get you started. A filter recipient plugin allows you to filter what recipients receive the email. If all recipients are filtered the email is disposed of. An injection  plugin allows you to interrogate the email before JMailQ adds it. In this plugin you can decide if the email should proceed or not. For example, if you have a component that always seems to be sending certain email and you don't want these emails sent, you can make that determination here without having to modify the component.

Delivery of emails is handle by a Joomla native CLI command which is run through a CRON job and whatever interval you wish.