Which is better to work with large data may more than 1000k records DataSet or DataReader

Hello,

In our project a windows service that sends emails  having DataSet that may contain minimum of  1000k records and we are sending emails  by fetching batch wise (10k records per batch) from the data set.

Due to this we are getting issue of memory in client server.

Can datareader is useful to work with more than 1000k recs?

Any better solution for this?