[TYPO3] Source for standard mailform

[Felix Natter]

hello,

where can I find the source for the standard typo3 4.0 mailform,
specifically the code for actually sending the mail?

I need to hack this so that a copy gets sent to the address
the customer entered. Using a mailform-extension is problematic
because all those I found use HTML-Templates and our customer
needs to be able to modify some parts of the form.

thanks,

-- 
Felix Natter