Actually, I really doubt this will be fast. In fact, it's likely to become a bottleneck itself. You're chewing up all the CPU's to try to load up the SMTP server with emails, leaving very little left for the SMTP server to use (if it's on the same machine), or leaving very little bandwidth for the SMTP server to send out the emails (if it's on a different machine).
Better bet is to use the envelope to specify all your recipients and send the mail out once. The SMTP server will only have to process a single message - which it will fork off and send on its own. For example, if 15 recipients are local, they can all be written through immediately without being choked by more incomings. And if another 5 are on another single host, the SMTP server can relay all of those in a single envelope, reducing both bandwidth and CPU time on both machines.
The only faster way than this is to send each mail to the proper machine directly - and, again, you need to send the mail to each host only once, using the envelope to address to multiple individuals at once on the same host to reduce the number of connections you need to make. But now you're writing your own SMTP server, which is unlikely to be a great use of your time.