We are currently developing a new email deployment system. We have the system currently configured so that it reads a record from the database and loads the email content and deploys it to the target. Now we want to move this over to multiple threads. That is easily done, except we then hit the email providers returning SMTP codes referring to "Too many connections", or "Deferred connection". The solution to this is to have a thread open up a connection to the email provider and deploy n emails and then disconnect. We have currently configured the application so that it will support these session based email deployments.

The problem is this, the database table has multiple email addresses in and they aren't grouped by email provider because that will show favouritism. We need to be able to retrieve a set number of, i.e. Hotmail, emails (@hotmail.com, @hotmail.co.uk, @live.co.uk) so that we are reducing the number of connections to Hotmail and reducing the risks of getting the "Too many connections" error. We are at the point now where we have gone round and round in circles trying to get a solution, so I thought I'd throw it out there and see if anyone has any ideas?

EDIT

I would like to stress that this application is not used for spamming purposes.