Blank page after step 3 of order is completed

I am encountering a blank page after an order is placed, shipping method is ok, order confirmation page is ok, but when you click the confirm order, it doesn't display the success page, it's a totally blank page (no source code), it does create an order and send a message to the admin.

I have tried the debug tool and it doesn't generate any code. The cache folder is writeable. I have some olde debug messages from previous use of the tool, but no current ones.

Finally, I turned off the send email in the configuration - email options and the order will complete.

I have to use smtp - it does send out messages to the admin regarding the order placement, it sends contact us messages and all the rest.

Any suggestions of why this particular page would fail w/ the email turned on?

All my other zen-cart installs on the same host work fine.