Swiftmailer sending 10,000 emails - getting an 404 error

[eluser]playaz[/eluser]
I am using the Codeigniter framework in a project - I have a script which reads an array and sends out over 10,000 emails using the SwiftMailer class.

One form which I have once submitted is supposed to send out each individual email, however it doesnt sent out all of them as after a period of time I get the following error:

404 Page Not Found. The page you requested was not found. - 500.shtml

The script itself does send out a few hundred emails but nowhere near the 10k email addresses stored within the system.

When I run the same script but sending out emails to a smaller set (eg 500 addresses) the script works as expected.

This is the script - would this be some timeout issue perhaps, I am unsure of the best way to fix this

http://pastebin.com/bxVbqQvN

PS Note - some info has been changed for security reasons (like IP addresses etc)