Why Does an E-mail System Require Servers?
An e-mail system consists of two different servers running on a machine. One is called the SMTP server, where SMTP stands for Simple Mail Transfer Protocol. The SMTP server handles outgoing mail.
The other is either a POP3 server or an IMAP server, both of which handle incoming mail. POP stands for Post Office Protocol and IMAP stands for Internet Mail Access Protocol.
In the simplest implementations of POP3, the server maintains a collection of text files — one for each e-mail account. When a message arrives, the POP3 server simply appends it to the bottom of the recipient’s file.
With POP3, once downloaded, email is removed from the POP3 server, with IMAP, the mail stays on the server.
Size of the server can be 2-10 Terra bytes. All email providers have several clusters of servers in various locations across the world. Each cluster has a few thousand servers.
Google server farms are all over the world: 19 in the US where they operate data centers, 12 in Europe, one in Russia and three in Asia.