IANAL, but: intent may be irrelevant in this case. The current fashion is to make so-called "strict liability" laws, especially in the area of "child protection". For example, in the UK, if there are child-porn pictures on your computer, then you are guily of an offence, regardless of how they got there. I don't know, but the same may apply in this case.
The beauty of this is that it allows the police to arrest people like this unfortunate person and put them in jail without all the tedious arguments about whether they intended do harm or whether it was an accident. A jury will be told "if he sent the message then he is guilty, even if it was a mistake".
Indeed, it is even possible for a policeman to force someone to do something against their will, and then arrest them for it. Google the case of "Winzar (1983)" if you don't believe me.