Yea he's pretty much right that you're not going to get a base+sport for anywhere near 25k lol probably not even a base, but I don't see them losing that much for a base+sport unless it's slightly used like high miles but never titled, or a demo car. His price of 31k is actually fairly accurate for the published invoice. You can realistically get to 28k on a base+sport 2010 if you're lucky.
I think there's a point where you can lowball all you want but it still has to be realistic enough for a dealer to actually accept. That email's pretty entertaining though, looks like you caught him on a bad day lol
TrueCar is a nice site that can give you an idea of how much people are paying.
2011 Nissan 370Z 2dr Coupe Manual - New Car Price Report
Also if you plan to finance, make sure you come in already pre-approved from your bank or credit union and you know your credit score, along with the current rates. That way you know if the rate they're offering is good or not. Some dealers love to say you can lower the rate by purchasing some extended warranty or some other BS extra, don't fall for that.
Half the battle is just getting the sales guy to agree to your price, the other half is getting through the finance department without getting ripped off.