When you send a mail message that can't be delivered, you get a bounce message back. If that can't be gotten to you, we have a double bounce.
Nowadays, virus scanning software causes a lot of unnecessary bouncing about when it tries to notify the sender that their message was blocked because it included a virus. The sender often doesn't really exist, or may be someone real who had absolutely nothing to do with sending the message: the return address is a forgery.
All of this comes from a basic design tenet of smtp mail: while mail CAN go from my computer directly to yours, it probably goes through other machines on its way. Those other machines are usually busy ISP's who aren't going to take the trouble to verify much about the reality of mail they are passing along. They accept mail from me at xyz.co, but nothing at all stops me from saying it's from email@example.com. The ISP's don't care, and probably shouldn't, but there are movements afoot to authenticate mail. Making such a system work will be pretty hard, but if it ever came to reality, I couldn't pretend to be firstname.lastname@example.org. If you got mail from joe, you could supposedly trust that it really came from him, and that it had been checked and verified every step of the way. Personally, I doubt that's going to happen, but we'll see.
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