Without seeing the code, I'm guessing that you didn't put your email address in quotes in mysite/_config.php.

Could you download the _config.php file from the online server and check if it's not corrupted, trunked or anything ?
You uploaded the site with a ftp client ?

Well, my english is not as good as I would like it to be. When I said "trunked" I meant "truncated"

I asked that because, last month, a truncated php file caused me some troubles. I guess it has been truncated because the ftp clients uploaded it with the binary mode instead of text mode.

It's not the case here.

However, your quotes are not good. For the email declaration you're using typographic quotes (curved ones), instead of the straight quotes from the key left of Enter key. (It's due to a copy-paste, I bet).

Yes, it's quite scary, even though the multinlingual doc is well done.
Be sure to backup everything before moving on, because the database migration may be a problem.
Bonne chance et bon courage.