Not necessarily true. If you have viewed it,or even opened it, even if recieved erroneously, you could be prosecuted. Convicted? Not likely, but the prosecution process alone is very painful.
You can be prosecuted for a lot of things, if there is a cowboy DA involved. It's actually not illegal to view child porn- if you walk into someone's house and they have child porn on their TV, you haven't comitted a crime. Knowingly producing, receiving or distributing child porn is illegal. Inadvertently or involuntarily seeing it is not.
A lot of this relies on the DA having some amount of discretion and common sense. Best Buy finds naked teen shots on the computer you sent in to repair? Prosecute. Someone sent an image on a cell that you deleted? Probably not even if they found out.
The law is supposed to get adults who take advantage of kids and the pervs who consume the resulting product (thus providing a market for the kiddie pornographers’ wares). DAs shouldn’t be bothering if no kids were taken advantage of by adults and there was no perv intent on the part of the accidental possessors.