A domain appears in this list if a call to Target is made from the domain. Often, you could see domains from spider engines, language translator sites, or local disk drives. If the listed domain is not one your team uses, you can click Delete to remove it.
So yes to answer your question it is kind of common to see unwanted domains in the hosts list.
For details and to restrict such domains you can create whitelists that specify hosts that are authorize to send mbox calls to Target.