Is it possible to throttle Joomla 1.5 Mass Mail so that it does not exceed the limit of 400 e-mails per hour?

This is not possible with the Joomla mass mail functionality. In order manage the number of email messages sent via Joomla for a specific time frame, you need a custom component to manage emails. It is advisable to to check the official Joomla extensions website for newsletter component that would suit your needs:

http://extensions.joomla.org/extensions/content-sharing/newsletter/

One of the most popular and widely used extensions we could recommend is Acajoom.

Joomla is a powerful software that is easy to work with once you have the right host to support you. Before you lose any more time with the wrong host, check out the SiteGround Joomla hosting offer.