system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does Instant Messaging differ from e-mail?
gpt
E-mail works in a similar way to the post office. You write a message and hit send, it gets passed off and the address read to see where it goes. It gets sent to that location where the address gets read and sorted into the correct mail box. Multiple clients and multiple servers all over the world just like mail boxes and post offices.    

Instant messaging is multiple clients and one server. Think of it like inter office mail. Everyone gives their letters to Charlie in the mailroom and he brings it up to their office. Charlie already knows where Carol is, so he can bring her a letter quickly. If who you want to talk to doesnt work in your building, Charlie can't deliver a message.