A mail server is a specialized piece of software which runs on a web server that co-ordinates email traffic and allows for the sending and receiving of email content.
A message transfer agent (MTA), also known as a mail transfer agent or mail relay, is software that transfers electronic mail messages from one computer to another via the Internet email system using SMTP. In many cases, the terms mail server, mail exchange, and MX host are also used.
Messages sent across networks, including any attached data files, are routed between mail servers (such as images, multimedia or documents). These servers frequently host email mailboxes. End users typically access this email through webmail or an email client provided by a hosting company.
Operations of a mail server
Mail is received by a message transfer agent (MTA) from another MTA, a mail submission agent (MSA), or a mail user agent (MUA). The Simple Mail Transfer Protocol specifies the transmission details (SMTP). When a message’s recipient mailbox is not hosted locally, the message is relayed, or forwarded to another MTA.
When an MTA receives an email message, it adds a Received trace header field to the top of the message’s header, creating a sequential record of MTAs that handled the message. The process of selecting a target MTA for the next hop is also described in SMTP, but can typically be overridden by configuring the MTA software with specific routes.
(Flow diagram showcasing the process in which email content is delivered)
An MTA operates in the background, while the user typically interacts with a mail user agent directly. Initial submission is distinguished by passing through an MSA—port 587 is used for communication between a MUA and an MSA, whereas port 25 is used for communication between MTAs or from an MSA to an MTA; this distinction is first made in RFC 2476.
The final delivery of email to a recipient mailbox is the responsibility of a message delivery agent for recipients hosted locally (MDA). To accomplish this, the MTA sends the message to the message handling service component of the message delivery agent (MDA). The Return-Path field is added to the envelop upon final delivery to record the return path.
A relay or filtering server will typically only store email for a short period of time, whereas other systems keep full mailboxes for email – in which case they usually support some way for end users to access their email via a Mail User Agent (MUA), or email client.
Types of protocols used by mail servers
This next section of this page goes over some common protocols used to achieve this.
POP (Post Office Protocol)
The Post Office Protocol (POP3) is a protocol that allows you to send and receive e-mail over the Internet.
Clients connect to a mailbox (maildrop) to retrieve, store, and delete messages from the server.
The protocol was designed to operate in both connected (online) and disconnected (offline) modes.
POP3 clients can also leave mail on the server after downloading it.
POP also offers the ability for an email client to download and store copies locally on the computer for archival purposes even after the content is deleted from the server.
POP is generally slower in comparison to IMAP however offers the ability to save content locally which makes it ideal for users who want to store all their emails to their local storage drives.
IMAP (Internet Message Access Protocol)
The Internet Message Access Protocol (IMAP) is a standard Internet protocol that email clients use to retrieve email messages from a mail server. IMAP is supported by almost all modern e-mail clients and servers. IMAP and POP3 are the two most commonly used standard protocols for email retrieval.
IMAP is quicker than POP however by default simply retrieves content from the server and doesn’t save it locally so if content is deleted from the server the local content when synced will also be removed from the email client on the users computer.
To mitigate this however IMAP archiving solutions exist in order to periodically download backup copies of email content locally whilst keeping the server clean and maintaining fast access times.
Examples of email clients
Email clients are the software programs you use on your computer to connect to a mail server and read your emails here are some common clients:
- Microsoft Outlook: Included in the Microsoft Office suite and is a very popular client on desktop computers running Windows.
- Mozilla Thunderbird: Popular free opensource mail client compatable with Windows, macOS and Linux.
- Apple Mail: Free application pre-installed on Apple’s macOS operating system along with iOS devices like the iPhone and iPad.
- Samsung Mail: The default email reader pre-installed on all Samsung devices including their smartphones and tablets.
Hosted/Cloud mail servers
In addition to these protocols also exist cloud based hosted solutions such as Gmail, Yahoo! Mail among many others which leverage these same technologies along with proprietary protocols and standards unique to the company.
For example Microsoft leverages proprietary technology called MAPI for it’s mail servers used some of their email services.
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Related links from our tech glossary:
Polluks, CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons
Message Transfer Agent. (n.d.). Wikipedia.Org. https://en.wikipedia.org/wiki/Message_transfer_agent