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SMTP & POP3 : What do they mean?
Simple Mail Transport Protocol
SMTP is the primary mechanism for moving mail round a TCP/IP network (eg the Internet). The protocol is designed to handle sites which have permanent connections to the network but has certain failsafe mechanisms built in to handle outages and disasters. These failsafes can be adapted to provide SMTP mail to dial up and other intermittent connections. Mail being sent is normally offered to a mail transport agent (MTA), such as sendmail which handles the delivery to the next link in the chain. The address of that next link is determined by checking the Mail Exchanger (MX) records for the host specified in the destination address. MX records are defined in the domain name service (DNS) records for the host in question. SMTP has proved itself over a long period of time to be very robust and able to handle major network problems and still deliver mail.
Post Office Protocol (v3)
POP3 fills the gap left by SMTP by providing a service more oriented to a single user. Mail is collected from the server rather than being delivered, as is the case with smtp. Mail is delivered by smtp from the Internet to a single server, or cluster of servers and held awaiting collection by the user. The most common implementation of the protocol assigns a single user ID to each mailbox. However a number of Internet Service Providers (ISPs) are providing a host based variant of the service, where all mail for anyuser@useraccount.domain is stored in a single POP3 mailbox for collection and filtering of the mail into user mailboxes is left for the customer to implement locally. This is really only a minor extension of the protocol where the user ID is not the user name but the hostname.
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