Step 1: Write the statement in logical form.
Let $G(m)$ mean "mango $m$ is good". The claim "all the mangoes are good" is $\forall m, G(m)$.
Its negation is $\exists m, \neg G(m)$, meaning at least one mango fails to be good.
Step 2: Match this to the given options.
"All are not good" and "no mango is good" both mean $\forall m, \neg G(m)$, a stronger claim than the negation, so they are not forced to be true.
"Some good and some not good" is only one specific case among many that satisfy $\exists m, \neg G(m)$, so it need not always hold.
Final Answer:
Only "there exists at least one mango that is not good" matches $\exists m, \neg G(m)$ exactly.\[ \boxed{\text{At least one mango in the basket is not good}} \]