[Exim] MailBomb Problem...

I am facing a problem where some users on the server are using php
mail() function to mail a list of users in taken from mysql db(around
5000 users, but not spam), but this is getting the system crashed due to
excessvie loads and if I limit the delivery load on exim than even
geninue mail is getting holded up.

so is there anyway to limit the number of emails or delay the mails from
the nobody user( which is the apach+php user which mails)

any help would be apperciated.
TIA

ewiz