I created a document library and configured "Incoming email settings" feature for it. A few days later, it was deleted accidentally. So I created it again. However, when I try to specify the previous email address for it, the error message popped up:
"Unable to assign this e-mail address to the list, because the address is in use"
Google leads me to here, but I don't really understand it. What I did, is giving it another email address, then change it to the original one.