As I said above, it’s not possible because the form sits on the only page, thus ignoring the contact page doesn’t do anything because the whole page gets cached 
[Edit] Guess it’s similar to the problem described here: Cache + Contact Form), even if that thread refers to the built-in mail function.
Caching would make the site so much faster on stupid slow 1und1 shared hosting, which doesn’t even support mod_deflate, arghh …