SInce you're using modules already and not mentioning custom dev, maybe you're looking for a module to do this for you? I haven't used any of his stuff but know that Onyak has something that'll probably do the trick for you.

I thnk that the biggest challenge regardless of how you get there is keeping your IP or domain clear of the blacklists and such. You didn't mention how many emails would be going at one time, but the quantity/quality of the send is something to give thought to so you don't get the mail server you're going to use put on any blacklists.

For this I wouldn't go past OpenWebStudio.

You will be able to do all of this quite easily and include it as a schedule item in DNN.  There's a small learning curve to get going but once you take that step I am quite sure you won't at all regret it!