PLANET CS-2001 - Manual (Page 467)

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How an Email is Processed
Composing and sending an email:
 Email delivery from an MUA to an MTA: Run a MUA client (email program)
and follow the instructions below:
 Apply the sender address and the domain name of outgoing mail server
(sender MTA), to the corresponding fields.
 Apply the recipient address and the domain name of incoming mail server
(recipient MTA), to the corresponding fields.
After finished composing the email with a MUA application, such as Outlook Express,
the user can deliver it to the specified MTA by clicking “Send”.
 If a MTA receives emails dedicated to internal accounts, the local MDA will
distribute the received emails to accounts based on account names.
 If a MTA receives emails dedicated to external accounts, it will automatically
forward them to the destination address. This functionality is called “Relay”.
 If a remote MTA receives emails sent from the local MTA, its MDA will
distribute them by the account names.
Receiving emails:
User uses MTA and MDA to check emails. If there is a new email, it will be
moved to the inbox of the user’s MUA application on the local computer. Meanwhile,
the MTA will clean up the remaining emails according to the MUA application
settings. If the configuration is set to keep the retrieved emails, then the read emails
will still be available when the recipient logs on to check emails.
Note：
1. Email protocols: SMTP & POP3
 Email delivery: signifies emails transferred from MUA to MTA and from MTA to MTA.
Nowadays, most mail servers use SMTP (Simple Mail Transfer Protocol) for outgoing
emails. By default, port 25 is assigned to the SMTP protocol.