Electronic mail uses the SMTP to transfer mail from one server to the next. Once the mail message has arrived at the destination mail server, the Point-of-Presence (POP) protocol is used to transfer it to the desktop. This protocol uses a spooling methodology, where a data unit is sent to a mail server, which then holds the data unit until the user accesses the server using an SMTP client application. The data unit is then downloaded to the user.

POP supports remote access of e-mail. It stores the mail message until the remote user accesses the mail server and requests mail delivery. Desktop hosts use POP, rather than fully functional SMTP, to access mail servers. With POP, desktop hosts do not need to provide the processing resources required by SMTP. POP is not as complex as SMTP and is only used by hosts accessing mail servers.

Mail servers can be configured to either save a data unit or discard it when it is downloaded. This of course oversimplifies the whole process for sake of brevity. The mail server tries periodically to send the mail to the user, usually until a counter expires, at which time the mail is either discarded or saved on disk.

If the mail cannot be delivered within a certain time, the server can be configured to send an error message back to the sender. This is a function of SMTP and not the underlying TCP or IP protocols. SMTP provides...