a 500mhz machine with 256MB of ram in it.

We let as many thread as it'll run (hundreds). But due to the speed of the machine and the ,method we use to send out, we initiate a new connect for each e-mail address rather than a batch deliver.

Our rate per hour reaches about 100,000 attempts an hour. We have a initial immediate delivery rate of about 78% consistently. We get of those 78% we get about 5-7% back from Bounce after accept. Of the non-immediate we get reject notices for bad addresses (various reasons) of about 20% of the remaining 22%.