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).

Presumably you would set threshold to a suitable value to keep the SMTP server going as efficiently as possible. After initially loading up the SMTP server with work to do, the script should spend most of its time waiting.