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PC Knowledge Base - Mail Server Components

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A mail server (or email server) is a computer system that sends and receives email. In many cases, web servers and mail servers are combined in a single machine. However, large ISPs and public email services (such as Gmail and Hotmail) may use dedicated hardware for sending and receiving email.
Every mail server that enables the sending and receiving of email with a local mail client has three separate software components:

MUA a Mail User Agent is the email client or email reader that accesses and manages a user's email.
A web application which provides message management, composition, and reception functions may act as an email client, and "email client" may also refer to a piece of computer hardware or software whose primary or most visible role is to work as an email client.
MSA a Message Submission Agent or Mail Submission Agent receives electronic mail messages from a mail user agent (MUA) and cooperates with a Mail Transfer Agent (MTA) for delivery of the mail. It uses ESMTP, a variant of the Simple Mail Transfer Protocol (SMTP),
Many MTAs perform the function of an MSA as well, but there are also programs that are specially designed as MSAs without full MTA functionality.
Historically, in Internet mail, both MTA and MSA functions use port number 25, but the official port for MSAs is 587. The MTA accepts a user's incoming mail, while the MSA accepts a user's outgoing mail.
MTA: a Message Transfer Agent or Mail Transfer Agent or mail relay is software that transfers electronic mail messages from one computer to another using Simple Mail Transfer Protocol (SMTP) The terms mail server, mail exchanger, and MX host are also used in some contexts.
Messages exchanged across networks are passed between mail servers, including any attached data files (such as images, multimedia or documents). These servers also often keep mailboxes for email. Access to this email by end users is typically either via webmail or an email client.
MDA: The Mail Delivery Agent or Message Delivery Agent is responsible for the delivery of e-mail messages to a local recipient's mailbox. It is also called a local delivery agent (LDA).
MRA a Mail Retrieval Agent retrieves or fetches e-mail from a remote mail server and works with a MDA to deliver mail to a local or remote email mailbox.
MRAs may be external applications by themselves or be built into a bigger application like an MUA. Significant examples of standalone MRAs include fetchmail and getmail.
The concept of an MRA is not standardized in email architecture. Although they operate like mail transfer agents, MRAs are technically clients when they retrieve and submit messages.
IMAP/POP3 Server: Manages users and their mailboxes as they check their email over IMAP/POP3 connections.

The Mail User Agent (MUA) is used to create an e-mail message. It is sent to a server, the Mail Transfer Agent (MTA). The MTA is responsible for transferring the message to a single server or collection of additional MTA servers, where it is finally delivered.
The client software (i.e., the MUA) communicates directly with a Simple Mail Transfer Protocol (SMTP) server, the MTA uses SMTP, which does nothing more than deliver messages from one server to another. Using DNS lookup, a sending MTA (MTA 1) then communicates with another MTA.

In the DNS zone, for the receiver address' domain, there will be an MX record. This is a DNS resource record that specifies the mail server of a domain. So, after the DNS lookup, a response is given to the requested mail server with the IP address of the recipient's mail server. In this way the 'to' mail server is identified.
The message is then delivered to a Mail Delivery Agent (MDA), where the receiving MUA downloads the message. An MDA and an MTA can reside on the same server, or on separate servers.

The system sends a request to find out the corresponding MTA of the recipient. This will be done with the help of the Mail Exchanger (MX) record.
If POP3 is used , then the whole email will be downloaded to the local computer and the copy at the server gets deleted.

If the protocol used is IMAP, then the email message is stored in the mail server itself, but the user can easily manipulate the emails on the mail server as in the local computer.
This is the difference when using both the protocols and this is how your email gets delivered. If some error occurred to send the email, the emails will be delayed. There is a mail queue in every mail server. These mails will be pending in the mail queue. The mail server will keep trying to resend the email. Once the email sending fails permanently, the mail server may send a bounce back email message to the sender's email address<

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