Hi Wizzle,
We're not currently using "Background Send", but it's available for plugins and other internal use. We were going to use it with Newsletter Builder but having thousands of copies of emails in the outgoing mail queue wasn't as efficient as generating the email content on the fly when we were sending them. So effectively you can ignore that option.
The first step is to find out what your hosts limits are.
Next, if you're using newsletter builder you can configure the max send speed with these settings at the top of the plugin:
$GLOBALS['BACKGROUND_SEND_MAX'] = 500; // max messages to send in each batch, eg: If a cronjob runs the script every minute and this is set to 30, you'll send approx 1800 emails an hour
$GLOBALS['BACKGROUND_SEND_WEB_MAX'] = 25; // max messages to send when calling mailer through web interface, browser sessions can timeout so set this low (example: 25)
$GLOBALS['BACKGROUND_SEND_DELAY'] = 0; // second to wait after sending each message
I'd just set them to slightly lower than the max. Other than that, for regular email usage (forum alerts, password reset emails, etc) you should be fine. But if you need to send a LOT of email very quickly then you'd either need to setup a cron as you suggest and throttle the emails to send slower and/or just find a 3rd party SMTP service and use them.
One issue with rate limited web hosts is that if you're sending a LOT of email and their limits are low it can take days just to send it all out.
Hope that helps!
Dave Edis - Senior Developer
interactivetools.com