We have a list of a large amount email address who we email daily through our Drupal Simplenews site.
We have setup where we use multiple server to balance the load and the database is on its own server. This is so there is room to grow the list.
We have the ability through these server to send mail very fast however when we do so gmail, hotmail etc all dont allow our mails through as they have a throttle which limits the amount of emails you can send to these guys and how fast.
when we slow the send down the mails get through to more people.
we are looking for an email marketing expert who can advise the best practice so we can send a far greater number of emails successfully to all providers including gmail and hotmail.
we are about to increase our lists in a big way and need all mails to be delivered and not get lost because gmail , hotmail etc throttle/block our send to them