I wrote:
I could of course background each call to sendmail.postfix but then I'd have to do my own concurrency management to throttle the number of simultaneous processes I had running that, and that seems to me to be exactly what Postfix should be doing for me.
I tried this approach, and it didn't help. I was able to put 20 messages in the outbound queue in 0.25 seconds, but it still took 10 seconds to flush the queue, indicating no sending concurrency.
