Kids understand junk mail.
Tell her, "We love your grandma, but I want you to know: I think it's rude to send other people junk mail. And that stuff she forwards to everybody seems interesting at first, but most of it is wrong, and pretty soon it'll be completely annoying. And then it's hard to tell them to stop.
"I don't want you to ever do it. If you see something that honestly some one, single, specific person might like, you can send it to them, but ONLY if you write them 3 paragraphs at the top to say hello and to explain why you think they would like it.
"I also don't like that Grandma just puts everybody's e-mail address in the big space. That means you all can see everybody's e-mail address. And if people click on "reply to all," then everybody gets MORE junk mail. I don't want you to do that kind of thing either. If you have to send an e-mail to more than one person, you have to think about whether they should all be able to see everybody else's e-mail. What if you want to send an e-mail to 3 girls from school. But one of them isn't very popular, and one of them is sort of popular with the mean girls. Then the mean girls could get the unpopular girl's e-mail address from their friend, and they could send nasty e-mails right to her in box.
"So you should use the BCC--that keeps anybody from seeing who else got the e-mail. If you want them to know that you're sending the e-mail to more than just them, you should write, "This e-mail is for Kristi, Marissa, and Krystal" right at the top.