SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving email. However, since it's limited in its ability to queue messages at the receiving end, it's usually used with one of two other protocols, POP3 or IMAP, that let the user save messages in a server mailbox and download them periodically from the server. Users typically use a program that uses SMTP for sending email and either POP3 or IMAP for receiving messages that have been received for them at their local server.
POP - Post Office Protocol(POP) is a standard mail server commonly used on the Internet. It provides a message store that holds incoming email until users log on and download it. POP3 is a simple system with little selectivity. All pending messages and attachments are downloaded at the same time. POP3 uses the SMTP messaging protocol.
Mail Relay - The use of a mail server from a connection outside of the immediate network. Open Mail Relays are the source of most originating junk email.
MX Record Mail Exchange Record. A DNS resource record type indicating which host can handle electronic mail for a particular domain.
Mail client - An email program that resides in the user's computer and has access to mail servers on a network.