Seven.
ennui
The word is rodents.
A six letter word for undeliverable mail is nixies.
nor
N- 5 letters:nannynasalnavelnoiseneedynylonnorthniecenastynoosenannyninthniftynoblenovelnaivenakednastynativenethernervenevernichenexusninnyninjanightnotchnudgenurse
niche
Seven.
ennui
Noodles
Steve-n
natio
vision
Nativity.
net
Nesbia
nonagon