So for those who are interested, here is a mod that allows you to throttle sending of messages.
Add to config/config.php file in the batch configuration section
# Throttle sending messages; indicates the number of milliseconds to pause
# between each message (millisecond is one-millionth of a second)
define("MAILQUEUE_PROCESSING_THROTTLE",300000);
You'll probably want to turn off the batch processing.
Modify admin/processqueue.php
find the following:
output("Sending $messageid to $useremail");
sendEmail($messageid,$useremail,$userhash,$htmlpref,$rssitems);
add after this the following line:
usleep(MAILQUEUE_PROCESSING_THROTTLE);
Be forewarned that this has not been tested much. I'm only using this in the command line interface mode, so be aware that it may have an impact if you're running the Process Queue from a browser due to the browser window needing to be open longer.
Please let me know if this works for you, or doesn't. I'm posting it here first to get some feedback, and if it looks good I'll join the developers list and propose the code as an added feature to the application.