I wonder if your email server is having problems with sending
them in loads?
I have a mailer system that sends out 5,000 per run and has
no problem. I
also had to build a delay into the loop so it sends in 200
email blocks.
Your code seems to be fine, except I am not sure how accurate
the
spoolEnable is and how it actually works.
My impression was, and I am sure I am mistaken, when looking
at the CF
admin, it was telling me that it looks for new emails to send
every 15
seconds, no matter how many are ready to go. So, if you
system is 'good' it
could have all 600 emails ready to go at once which could
choke your
mailserver. Again, I might be wrong, I am not the System
Administator... 🙂