make the following edit:
/includes/functions/functions_email.php
At line 278 you'll see this:
Code:
/**
 * Send the email. If an error occurs, trap it and display it in the messageStack
 */
Add this above it:
Code:
var_dump($mail);
die('end of email details');
Then send a test email.

You'll see just a white screen with much of the email details.
Right-click on the screen and select "View Page Source" (or "View Source" in IE).
Then copy+paste the entire source here.