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) or make PHP use a remote server than the local sendmail ? any help would be apperciated. TIA ewiz