The Iranian import partners are:
Germany 12.8%, France 8.3%, Italy 7.7%, China 7.2%, UAE 7.2%, South Korea 6.1%, Russia 5.4% (2004)
The Iranian export partners (petroleum 80%, chemical and petrochemical products, fruits and nuts, carpets) are:
Japan 18.4%, China 9.7%, Italy 6%, South Africa 5.8%, South Korea 5.4%, Taiwan 4.6%, Turkey 4.4%, Netherlands 4.1% (2004)
However it is false that Iran is one of Italy's main export markets. This market is too small for the quite powerful Italian economy that it would be irreplaceable. Same thing with Germany and France. The main problem I see are the Japanese who buy a big portion of their oil from Iran. If those countries (Japan, Germany, France, Italy) would join into a complete embargo (no buy - no sell), Ahmadinejad would be in really bad trouble.
I doubt that it will happen, since the economical price to do so is really high. Anyway it would be better than any war. The question is, how valueable Israel will be to those 4 nations.
I'm sorry about not being more specific the first time. Nearly half of the French are anti-Israel. Most of the Japanese don't care one way or another about Israel. Most Italians and Britons who have opinions on the matter are anti-Israel.