What do you mean exactly by illegal? Nobody's going to jail for sending an email outside of working hours, even in France.

Edit: actually, for anyone wondering, the law is that a >50 employee's company should document it's own recommendation about how not to harass your colleagues during out-of-work hours. When fully applied, the worst case scenario would be that, as an employee, you can officially (ie. under a legal base, not legal right, but IANAL) ask your employer to have workaholic colleagues to step down on their out-of-hours emails. Nothing more, nothing less, very very far from "in France it's illegal to send a work email outside of the working hours".