In case your Joomla! application is using the AcyMailing extension you will be able to configure it to send less than 400 messages per hour, by following the steps below:
Access your Joomla! Administrator area and go to:
Components -> AcyMailing -> Configuration -> Queue process.
In the Manual send process configuration enter the following values:
Send 40 e-mails and then wait 6 Minutes before sending an other batch.
More details about the AcyMailing configuration can be found in this article.
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