You're probably overloading something.. the cgi engine maybe?
I don't use any CGI, and i was watching "top" while i did it and no load averages changed.
I don't know a thing about it but off hand I would say too many at once. Cut it down send half at a time, if that doesn't work try a third and so on.
i tried that, i went as far as wring a script that would send it in chucks of 3000 emails at a time and it still did it.