Well, most of the time it works fine, and the newsletter perks along nicely, going out to tens of thousands of readers each week. But in some cases, gremlins strike. When that first happened, I assumed it was an error on th e sender's part. For example, the Microsoft Network uses two e-mail systems. Other ISPs sometimes have similar aliases.
Private companies do it, too.
Actually, there's no intent to deceive anyone. The extra information is there to provide better routing through complex systems, allow for e-mail loads to be shared among multiple machines, and so on. In fact, it'd be a non-issue if it worked properly all the time. Then, aliases would be totally transparent in both directions, and mail would always get through. But clearly, that doesn't always happen.