Let's follow the route of a typical letter through the postal system:
You write your letter and drop it into the post box.
The postman empties the postbox and takes all of the letters to the local sorting office.
Someone at the local sorting office looks at the address written on your envelope and puts it into an appropriate bag for delivery to another sorting office nearer to your correspondent.
The bag gets delivered to the new sorting office.
Someone takes your letter out of the bag, gives it to a postman to deliver to your correspondent's house.
The postman delivers the letter.
It's a pretty simple system and it works. So, back to the electronic version, which goes something like this:
You write your message and click "Send".
Your computer takes your message and transfers it to an outbound mail machine (usually provided by whoever supplies your network connection.)
The outbound mail machine looks at the address on your message envelope and transfers your message to the machine that looks after your correspondent's electronic mail.
This new machine looks at the message and delivers it to your correspondent's mailbox.