Every e-mail has its own server(服务器).When you send the e-mail to others ,you only need to write the receiver's e-amil address.The server will help send to e-mail to your friend's e-mail server.
This is just my opinion, if there is something wrong ,please help me correct it.TKS!
Firstly, the software scans the outgoing message to make sure it doesn't contain any viruses. Once the message has passed the test, it transmits to a SMTP (Simple Mail Transfer Protocol) server. The SMTP server searches the header of each email to find out the email addresses of the recipient and then transmits the email if it receives a positive response.
Similar as the Web Pages, every Email has its own address. After scanning the viruses, the sever will send out the Email to the exactly address you want.