In the US I believe normal mail cannot be substantially modified in transit, and that this is a Federal offense. For instance for a postal worker to open letters and insert cartoons because it was April Fool's day he would not only lose his job but also be subject to prosecution.
Certainly an email server is in some way exempt as it is possible for software to add headers to an email, but if the server modifies the message and the manner of communication substantially, there might be serious legal ramifications to this practical joke of Google. Especially since this joke impeded the response in a way similar to scratching out the return address of a postal letter.
The very fact that Google thought it worthwhile to do such a thing with personal communication absolutely boggles the mind. Any responsible organization would be insane put their email on their servers after such a stunt.