#This cron task will run the cbsubs mailer rules every 5 mins and put msgs in the outbox

That is entirely too quickly. My guess is you're causing mailer processes to overlap. Ideally you should only be processing the mailer queue once or twice a day at most. The only way it'd be ok to run it that often is if you run "Url of cron task to process mailers rules" every hour to put users into the queue then run "Url of cron task to process mailqueue" at the end of the day to process the queue, but that isn't really necessary as running "Url of cron task to both process mailer rules into queue and then to process mailqueue" (the URL you're currently using) every 24 hours should be sufficient.

Additionally you can use "Maximum messages per customer (0=unlimited)" to directly limit the number of times the mailer is sent to them. "But do not send message if it's later than" can be used to limit the date check range as well.