AirLive ES-6000 - Manual (Page 220)

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Take sending and receiving mail for example to illustrate the process of mail transmission.
When user wants to send a mail:
„ Send mails to MTA via MUA. While composing email with MUA, make sure to follow steps below :
1. Enter the sender email address and outgoing mail server (namely the sender MTA )
2. Enter the recipient email address and incoming mail server (namely the recipient MTA for receiving
mails from external host server)
After user finished composing a mail with MUA, such as Outlook Express, mail can deliver to the appointed
MTA by clicking on Send.
„ When MTA receives mails from itself, MDA will distribute received mails to each mailbox according to
account names.
„ When receiving mails of recipients from other MTA, it will automatically forward these mails to that MTA.
This function is called Relay.
„ A distant MTA is able to receive mails from local MTA. The distant MDA will take over the received mails
and store them waiting for user to log in to download them.
The process of receiving mails:
A remote user inquires his MTA for checking new incoming mails, if MDA detects new mails, mails will be
sent to user’s MUA. Meanwhile, MTA will clear up the mailbox or keep those mails according to the MUA
settings.
Email Protocols: SMTP & POP3
1. Sending Mails: signifies mails transferred from MUA to MTA and from MTA to MTA. Nowadays
most mail servers use SMTP (Simple Mail Transfer Protocol) for outgoing mails. And the port 25 is
assigned to SMTP protocol.
2. Receiving Mails: signifies MUA using POP (Post Office Protocol) to communicate with MTA so as
to read or download mails in user’s mailbox. Currently, POP3 (Post Office Protocol version 3) is the
most popular protocol for incoming mails. And the port 110 is assigned to POP3 protocol.
Usually MTA needs at least two protocols, namely SMTP and POP3, to receive and send mails. As long
as both MUA and MTA support SMTP and POP3 protocols, mail servers will be able to interact with
each other.