The e-mail sending program that I made, can send ~50 e-mails per second. But because of this problem I had to slow it down to 1 email every 3 seconds. Our admin told me to slow it down by this much until we find out how to fix this problem.

They are planning to get a faster internet connection (something optical), but I don't think it's gonna solve the problem because that still does not stop other servers to block our IP...