Can't Reach Server fix? Hamachi.

My friend is trying to join my Tekkit server (a modded Minecraft server).
But when he tries to join it says "can't reach server" or something along those lines.
I can join it without any errors, but he can't. Does anyone know how I can fix this so we could play together?

my friend has the exact same problem. It's the server fault, but im not sure what's causing it.