Largest postage impossible with 5 and 21 stamps

You have unlimited stamps worth $5$ and $21$. What is the largest total value you cannot make exactly?

79. For coprime $a,b$ the largest non-representable value is $ab-a-b$ (Frobenius), here $105-26=79$. Concretely, the cheapest way to reach residue $4\pmod5$ is $84=4\cdot21$, so $79$ is the largest value in that class that is unreachable.