email not created if the script goes too fast !

Hi all !

I've got a very weird problem !

I was trying to make my PHP script to send emails to every subscribed
member to go faster (each email takes 1 or 2 seconds to be sent !) so
I tried to send my emails directly with Sendmail instead of the
function mail(), with the option "-odb" (delivery in background).
With this option, the script was really faster, but I noticed that all
the emails were not delivered !

I wrote a simple PHP with just a loop that sends 100 emails of 20 kb
each, with Sendmail and the delivery in background, and the results
are very weird !
- normal script (just the loop) : only 50 emails delivered (76.30
seconds)
- script + sleep(0.5) after each email : 78 emails delivered (83.33
secs)
- script + sleep(1) after each email : 100 emails delivered (149.44
secs)

I also noticed that if the email gets bigger, fewer of them are
delivered !

I looked in the queue directory of Sendmail: nothing !


Is it possible that if a script goes too fast (I don't think 76 secs
for 100 emails is very fast, but ...), an email is not correctly sent
?!

Has anybody an idea ?!
Thanks
Krystoffff