Most efficient to mass mail - php

HI there,
I have built a newsletter for personal use.

what i currently do is that i mail 100 emails per batch then sleep 5 min.
Therefore the max to send in one go is a 1000. i have cron jobs that kicks in every 15min. I mark my newsletter as being active so other cron cannot disturb the current one as (as 10 batches will take approx 50 min).

I wanted to know if i am on the right track. as i also need to keep my db connection open so it is a constant ping to db every second.

I dont know whether there is a better method for that, as sometimes my newsletter break and i think it is on the db connection!

If anybody can help me in that matter i will really appreciate it.

Thanks a lot.

Regards,

Jerome