Yes, the generating in parallel was something we discovered as well. Although the max injection rate (SMTP submits) we could get out of a process was 250,000 per hour, we discovered that doing 2 at a time didn't slow down this rate much. We were thinking of running 4 at a time on each server. But I'll trust and suggest your number 8 - that makes sense. If I can linux to submit at 250,000 by putting it on EMC array (which is like ram disk), we'll be in business with multiple processes.
On a side note, the 250,000 was up from 125,000 when I made the following code change on Net::Cmd ->
line 476__ $main::ignore_response ? 1 : ($cmd->response == CMD_OK);
I noticed alot of delay in waiting for the response when a mail was finished submitting to postfix. I set ignore_response as a global in the sending program.
Thanks for your help and confirmation of the multiple-processes strategy.