Looking at the above diagram, you see it has three major components: user agents, mail servers, and Simple Mail Transfer Protocol (SMTP) . A sender wants to send an email to a
recipient. User agents allow users to read, reply to, forward, save, and compose messages.
When the message is completely composed, the sender's user agent sends the message to
their mail server, where the message is placed in the mail server's outgoing message queue.
When a recipient wants to read a message, their user agent retrieves the message from their
mailbox in their mail server.

In the late 1990's, graphical user interface user agents became popular allowing users to view
and compose multimedia messages. Mail servers from the core of the e-mail infrastructure,
each recipient has a mailbox located in one of the mail servers. If the sender's server cannot
deliver the message to the recipient's server, then the sender's server holds the message in a
message queue and attempts to transfer the message later; usually every 30 minutes or so. If
the message has not been delivered after several days, the server removes the message and
notifies the sender with an e-mail message.