What exactly is a bad address?
Excellent question, and one that we have never specifically answered in writing so it's a good exercise to do.
218 Gadsden Hwy Birmingham, AL 35235
[house number invalid] 214 Gadsden Hwy Birmingham, AL 35235
[house number missing] Gadsden Hwy Birmingham, AL 35235
[street/zip mismatch] 218 Gadsden Hwy 35211
[missing state or zip] 218 Gadsden Hwy Birmingham
[street misspelled] 218 Catsden Hwy Birmingham, AL 35235
[street misspelled] 218 Gads Hwy Birmingham, AL 35235
VALID ADDRESSES (but not deliverable):
You can also have this same address and it could be vacant or marked as inactive. That would make an otherwise valid address, not deliverable.
So, those are just a few examples. Indeed, there are many ways that a single address could be bad. It could be incomplete or just plain wrong.