You have to configure your phpList script to send less than 400 messages per hour. You have to open the phpList config.php file and change the parameters as shown below:

# Send a batch of 360 messages per batch period:
define("MAILQUEUE_BATCH_SIZE",360);

# batch period is set to 3600 seconds (=1 hour):
define("MAILQUEUE_BATCH_PERIOD",3600);

# Pause between messages (in seconds) to avoid overloading the server:
define('MAILQUEUE_THROTTLE',1);

The configuration above will force the phpList to send the messages in batches of 360 messages per hour.

Additional information regarding this matter can be found at:

http://docs.phplist.com/PhpListConfigSendRate