You will probably want to have table entries for sent emails. That way it serves as an audit trail if processes go down or somehow fail.
Suggest you look at doing this with an elastic cloud database like MongoLab, MongoHQ or SimpleDB. (Mongo-based services make it easy to extend the schema for new email entries.)
If you do that, then a cloud worker queue like SimpleWorker can make it easy to send out lots of emails concurrently or in batches to get around any rate limits. (full disclosure: I work at Iron.io/SimpleWorker)
You're taking a good approach to bundle multiple email sends into a single worker task to amortize the worker setup costs. With an elastic cloud worker system, you could have master workers come off schedule and then queue up a number of slave worker tasks, each with a set of users to send.
With table entries, you can then go back through the data tables and address any emails that failed or didn't go through.