Okay, you want to send 20 emails in a 30 minute period.
Amend your config.php file as follows. (if it doesn’t already include this code*, copy/paste the code below)
# define the amount of emails you want to send per period. If 0, batch processing
# is disabled and messages are sent out as fast as possible
define(“MAILQUEUE_BATCH_SIZE”,20);
define the length of one batch processing period, in seconds (3600 is an hour)
define(“MAILQUEUE_BATCH_PERIOD”,1800);
to avoid overloading the server that sends your email, you can add a little delay
between messages that will spread the load of sending
you will need to find a good value for your own server
value is in seconds, and you can use fractions, eg “0.5” is half a second
(or you can play with the autothrottle below)
define(‘MAILQUEUE_THROTTLE’,1.5);
============================
*It’s actually stored in config-extended.php but for it to work code needs to be in config/config.php file.