The word is rodents.
A six letter word for undeliverable mail is nixies.
Seven.
nor
nixie
newt
anil
N- 5 letters:nannynasalnavelnoiseneedynylonnorthniecenastynoosenannyninthniftynoblenovelnaivenakednastynativenethernervenevernichenexusninnyninjanightnotchnudgenurse
Noodles
natio
vision
Nativity.
net
Nesbia
nonagon
Niagara
The word is rodents.