edit: I've checked up a bit. There is such a thing as "oral contract" in French law but it's very strict and limited to certain transactions.
The legal definition of contract doesn't say if it must be written or not (http://www.legifrance.gouv.fr/affichCodeArticle.do?cidTexte=...).
It's extremely rare (read: never) to start a work without a signed fax or letter which basically says "ok, go ahead for that price".
That goes from a plumber who's going to fix your sink to me paying a designer to work on a web site. I guess in some cases an e-mail has got a valid proof, but it depends on the price of the transaction. You can confirm a 1,000 € job by e-mail, but I guess a 100,000 € transaction needs more.
What you need is a proof that the agreement occurred between the two entities.
When giving someone the right to sell your product, I'm quite certain than an oral agreement or even an e-mail is insufficient.
A printed e-mail sent by fax, let's just not talk about it. ;)