This page details some definitions for users who may be unfamiliar with them. All definitions coutesy of whatis.com.
Domain Name System (DNS)
The domain name system (DNS) is the way that Internet domain name are located and translated into Internet Protocol addresses. A domain name is a meaningful and easy-to-remember "handle" for an Internet address.
Because maintaining a central list of domain name/IP address correspondences would be impractical, the lists of domain names and IP addresses are distributed throughout the Internet in a hierarchy of authority. There is probably a DNS server within close geographic proximity to your access provider that maps the domain names in your Internet requests or forwards them to other servers in the Internet.
MAPI
MAPI (Messaging Application Program Interface) is a Microsoft Windows program interface that enables you to send e-mail from within a Windows application and attach the document you are working on to the e-mail note. Applications that take advantage of MAPI include word processors, spreadsheets, and graphics applications. MAPI-compatible applications typically include a Send Mail or Send in the File pulldown menu of the application. Selecting one of these sends a request to a MAPI server.
MAPI consists of a standard set of C language functions that are stored in a program library known as a dynamic link library (DLL). Developers who are using Microsoft's Active Server Page (ASP) technology access the MAPI library by using Microsoft's Collaboration Data Objects (CDO). The CDO library comes with Microsoft's Internet Information Server (IIS). MAPI functions can be accessed by Visual Basic developers through a Basic-to-C translation layer.
Eudora, one of the most popular e-mail programs, includes a MAPI server. To activate the Eudora MAPI server, select Options from the Tools pulldown menu and then double-click on MAPI and specify whether MAPI should always be active or whether it should be active only when Eudora is running. Microsoft Exchange and possibly some other programs may require that you turn the MAPI server off when they are in use.
POP3
POP3 (Post Office Protocol 3) is the most recent version of a standard protocol for receiving e-mail. POP3 is a client/server protocol in which e-mail is received and held for you by your Internet server. Periodically, you (or your client e-mail receiver) check your mail-box on the server and download any mail. POP3 is built into the Netmanage suite of Internet products and one of the most popular e-mail products, Eudora. It's also built into the Netscape and Microsoft Internet Explorer browsers.
An alternative protocol is Internet Message Access Protocol (IMAP). With IMAP, you view your e-mail at the server as though it was on your client computer. An e-mail message deleted locally is still on the server. E-mail can be kept on and searched at the server.
POP can be thought of as a "store-and-forward" service. IMAP can be thought of as a remote file server.
POP and IMAP deal with the receiving of e-mail and are not to be confused with the Simple Mail Transfer Protocol (SMTP), a protocol for transferring e-mail across the Internet. You send e-mail with SMTP and a mail handler receives it on your recipient's behalf. Then the mail is read using POP or IMAP.
The conventional port number for POP3 is 110.
Simple Mail Transfer Protocol (SMTP)
SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail. 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 Internet Message Access Protocol, that let the user save messages in a server mailbox and download them periodically from the server. In other words, users typically use a program that uses SMTP for sending e-mail and either POP3 or IMAP for receiving messages that have been received for them at their local server. Most mail programs such as Eudora let you specify both an SMTP server and a POP server. On UNIX-based systems, sendmail is the most widely-used SMTP server for e-mail. A commercial package, Sendmail, includes a POP3 server and also comes in a version for Windows NT.
SMTP usually is implemented to operate over Transmission Control Protocol port 25. The details of SMTP are in Request for Comments 821 of the Internet Engineering Task Force (IETF). An alternative to SMTP that is widely used in Europe is X.400.
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