When you send an email message, it is first broken up by the Internet's TCP protocol into IP packets. Those packets are then sent to an internal router that examines the address. (An internal router is inside your network, rather than out on the Internet.) Based on the address, the internal router decides whether the mail is sent to someone on the same network or to someone outside the network. If the mail goes to someone on the same network, the mail is delivered to them.
