A mail user agent, also known as an email client, is a computer program that lets you send and receive an email by interacting with the user. Well, MUAs are part of the Simple Mail Transfer Protocol (SMTP) system that creates email messages and sends them to a Mail Transfer Agent (MTA). MUAs can come in the form of software apps or webmail services.
How Does An MUA (Mail User Agent) Work?
When “A” wants to send an email message to “B,” he opens the Mail User Agent of his choice, writes his message, and clicks “Send.”
The Simple Mail Transfer Protocol (SMTP) server is contacted by the MUA of person “A.” First, the MUA of “A” tells the SMTP server who sent the message and to whom it should be sent. Lastly, it sends the message’s headers and main body. When the SMTP server accepts the message and agrees to send it to its final destination, the MUA tells “A” that the transmission was successful.
The SMTP server stores the message and tries to send it where it needs to go. For example, if person “B” is far away from where the mail came from and has a different email provider than the person “A,” the SMTP server of the email provider “A” decides which server handles the mail for the “B” domain. The server (Domain Name Server) looks up this information in DNS. DNS tells the mail server where to send mail for a certain domain.
Person “A’s” email provider’s SMTP server connects to the other SMTP server. It sends the message to him in the same way that the “A’s” mail program did before. The server can talk directly to the final SMTP server that handles person “B’s” mail, or there can be a chain of intermediate servers that send the message to its final destination. How the message will get from “A” to “B” depends on “B’s” email provider and, to some extent, on how well “B’s” Internet connection is working at the time the message is sent. For example, if “B’s” main mail server goes down, mail may be sent to another server for a while. Or “A’s” mail server may stop the message and then try to send it again from time to time.
The message from person “A” finally gets to the last server, which stores the email for person “B.” The message will stay there until “B” connects to his mail server to get it. Then, when “B” clicks “Retrieve mail” in his MUA, the mail client talks to the server and downloads the mail. Most of the time, this last download is done through a protocol called POP3 (Post Office Protocol) or IMAP instead of SMTP (Internet Message Access Protocol).
Difference Between A Mail User Agent (MUA) And Mail Transfer Agent (MTA)
MUA (Mail User Agent)
An MUA, or Mail User Agent, is a piece of software, a program, or an app that lets a user, at the very least, write and read email messages. People usually call them a mail client. However, many MUA programs have a lot more features. Some of these features include getting messages by using the POP and IMAP protocols, setting up mailboxes to save messages, and helping to show new messages to a program called MTA, which will send them to their final destination. These programs, also called MUA apps, can have a graphical interface like Mozilla Mail or a simple text-based one like Mutt or Pine.
MTA (Mail Transfer Agent)
Using the SMTP protocol, an MTA (Mail Transfer Agent) program or piece of software moves e-messages from one machine to another. One email can go through more than one MTA before reaching its final destination. Most users don’t know these agents exist, even though every message goes through at least one MTA.
Even though sending messages between machines seems to be a simple process, choosing whether or not a certain MTA agent can or should accept a message for delivery to a remote host is a very complicated process. Also, because spam causes problems, the configuration of an MTA is set up, or the network access of the system that runs it usually limits or restricts the use of a specific MTA.
There are also several larger, more complicated MUAs that can be used to send mail. But this action should not be confused with the real functions of these agents. For example, people who don’t run their MTA but still want to send outgoing messages to a remote machine for delivery must use a feature in the MUA that can transfer the message to an MTA for which they have permission. But the MUA agent does not send the message directly to the mail server of the final recipient; the MTA agent does this.
Mail User Agent Examples
These are some kinds of MUA agents:
- Pine
- Evolution
- Thunderbird
- Kmail
- Lotus Notes
- Yahoo!
- Microsoft Outlook.com and
- Gmail