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:

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

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