Mail arrives from a friend, or at least somebody you know well enough to have them in your address book. But it isn’t to you, it’s a mass blast to everybody in their address book. And they didn’t even use a cc: list, they created a little mailing list. The return address is a person’s email, but you, you’re just a listing.
Ok, first off, this is rude.
Second, it makes the return email address useless. It used to signify an individual that you might correspond with, but now it means you’ve got junk mail, or at least bulk mail.
If you do this to me, it means that any personal mail you do send will either get auto deleted by my junk filter or it will get moved into the folder for noisy bulk mess that I rarely read.
But then again, I realize that people who do this are burning the personal friendship for the sake of a work project. When you join their bulk mailing list it means you’re dead to them but might be good for something after all.