MD5 Hash outputs not produced Are there any hash outputs not produced by the MD5 hash function? I need to use it as a placeholder for an empty hash while comparing two hashes.
Not provably. Maybe, since it's not a permutation, but you couldn't verify it practically.

Just use all-zeroes: it's easily recognizable as a null value, and thanks to pre-image resistance, no-one knows what hashes to that.