It's inevitable that people will be annoyed when they receive emails in a mailing list that they haven't subscribed to, but companies still persist in sending them.
Ideally, people should have to opt-in before they become a member of a list. However, some companies talk themselves into believing that their customers would opt-in if they had the choice, because the list is so useful, so they remove the need to opt-in at all. For example, Legacy Family Tree is an excellent genealogy program, but whenever you download a program update (even if you've paid and registered your copy) you are automatically resubscribed to their mailing lists. There's no opt-out option, and it doesn't matter how many times you've unsubscribed in the past, they put you back on their list to receive regular advertisement emails.
When you go to their web page to unsubscribe, you have to select which mailing list to unsubscribe. Since you didn't subscribe in the first place, it's difficult to know which to pick, so I selected all of them. Then, bless 'em, they sent me an error email saying that I wasn't subscribed to one of the lists I tried to unsubscribe from. Maybe they'll resubscribe me automatically to fix that error.
I wonder what the developer was thinking when he created that email? Hmmm, this person tried to unsubscribe because he doesn't want to receive any more emails. But he can't unsubscribe because he's not subscribed to this list, so I'd better send an email to let him know why he's not been unsubscribed to this list...
Moral of this story: use opt-ins for mailing lists. If you can't then allow people to opt-out. If you can't do either, then don't do a mailing list.