I came across this story about exchange server.

The story in a nut shell:

If a mailing list is a part of another mailing list, un-subscription of the former sends an unsubscribe message to people in the second group. People in the second group all send emails, please do not “unsubscribe me” message to the whole list eventually, taking down the mail server!

The author of the post put it in better words, read it for some fun!