The JMailQ component is a Joomla mail manager. It captures all mail generated by Joomla and its extensions (provided they use the Joomla JMail interface) and queues them for delivery.
JMailQ will meter out the emails based on the frequency you define: maximum per minute, per hour or per day; maximum in any one send cycle; maximum execution time per cycle; minimum time delay between emails. Some hosting companies actually drop any emails that exceed their limits, others simply charge extra. If you are using a hosting provider that enforces a volume limit on the number or frequency of emails over time, this extension is for you.
JMailQ also offers another benefit, reduced user wait time. If you are operating a forum or other type of social activity where the actions of a user can cause Joomla to send multiple emails, you may have noticed a delay between the time the user performed the action and when the system responded to them. During this time Joomla is sending the emails that were generated as a result of the action. Consider a forum where you have hundreds of subscribers to a topic and each subscriber receives a notice of a new topic. The person posting the new topic must wait until the system generates and sends all of these emails before the system comes back to them. This problem is exacerbated when you use an external mail sever. Users can get very frustrated and often do more damage by resubmitting etc. With JMailQ the emails are all captured, stored, and sent independent of the user so the system returns control to the user almost instantaneously.