550 Requested action not taken: mailbox unavailableSMTP Server error (code 0x800CCC79) You might’ve seen messages like that before and wondered what in the world of 0’s and 1’s they were talking about. Or you might’ve felt a false sense of your own technical prowess one day (big mistake!) and decided to dabble in your Gmail or Outlook settings (potential catastrophe), only to be greeted with gibberish and jargon that was so alien to you it might as well have been written in assembly (but you decided to poke around anyway, didn’t you?!)
Well, be befuddled no more! In this article we explore what email servers are and how they work so that the next time you visit your email settings page you’ll know what’s what (though we still don’t recommend mucking it up by experimenting).
Email is so ubiquitous in our lives today that we seldom think about how it gets from here to there. You will no doubt have noticed that there are several components to the email system. There’s you (the sender) and what you’re composing (the email message itself), for which you will need some sort of tool to compose (the email client). You’re definitely sending that email to someone, so there’s a recipient, and you’ll need to tell the message how to find the recipient (through an email address). These parts are obvious.
Less obvious is the protocol that is employed in sending your message, email servers that handle that sending, the path the message takes from origin to destination, email servers that handle receiving and storing the recipient’s mail, the storage area (or mailbox) of the recipient, and the recipient’s own email client that allows them to retrieve and view the message.
Almost everyone who uses the internet has an email address. And while each address is unique, they all share a common structure: name@domain; with an example being: firstname.lastname@example.org.
When you send an email to an address, your email client (also called the mail user agent, MUA) first sends it to an outgoing email server (aka mail submission agent, MSA) using a protocol known as Simple Mail Transfer Protocol (SMTP). The server then uses SMTP to transfer the message from that first server across the internet until it reaches the recipient’s email server (aka mail delivery agent, MDA). The MSA is the first type of main email servers and is known as the SMTP server.
Finding the recipient’s mailbox happens in the same way the post office finds the address on an envelope: the sending post office would consult a map to find the country and city and send it there. Then the local post office uses a local map to find the neighbourhood, street, house, and dispatches the postman to the proper location. These maps represent the domain name system (DNS), which contains servers that act as directories showing where each domain name “lives”. The server starts by searching for the top level domain servers (.com, .org, etc.) and asks them where the domain name servers are (@bmail.com, @gmail.com, @outlook.com, etc.). Once it finds those email servers it asks them where the specific name is (alice@, bob@, kingoftheworld@, etc.).
In order to complete the above operations, the sending server consults what are known as mail exchange records (MX records) and address records (A records), which are parts of the DNS. The MX record shows where the messages in a domain should be delivered, while the A records give the IP address of the server for the domain. So the MX records says that mails ending in @bmail.com go to email servers at mail.bmail.com, and the A record says that the IP address of that server is 18.104.22.168.
Once the recipient’s domain is found, the message is sent to a final server containing the recipient’s mailbox storage, which uses a protocol called the Post Office Protocol (POP, currently version 3, so POP3). The POP3 server is the other type of main email servers. The recipient’s mail client will connect with the POP3 server to retrieve the message.
The most prevalent protocol for email retrieval is POP3. It has been in use since the 1980s and was developed in an era where internet connectivity was intermittent at best and only one client at a time could connect to a mailbox. The client would request new messages on connection. It was originally set up to retrieve the messages from the server to the client, store them in the device’s local storage, and then delete them from the server. Obviously they could only be accessed from that device later on. An option to maintain them on the server currently exists.
A newer protocol for retrieval, the Internet Message Access Protocol (IMAP, currently at version 4; IMAP4), was developed to overcome the shortcomings of POP3. It takes into consideration the constant network connection available today, delivering messages to the client as they arrive. It can also support multiple simultaneous connections, with protocols for recording and syncing the changes applied by different clients. IMAP4 does not delete the messages from the server but makes a local copy, allowing mail to be accessed from any device with an internet connection. IMAP supports categorizing the messages into folders and giving them tags like “read”, “answered” or “deleted” Still newer protocols, POP4 and SMAP, are under development as experimental alternatives to IMAP. They are created for greater simplicity in handling message retrieval.
You’ll notice that the client and the server are two separate entities. Clients can be desktop applications, like the Outlook app and Thunderbird, and can also be accessed through web browsers, in which case it’s called Webmail (like Gmail). Webmail clients have the advantage of being accessible from any device with an internet connection, whereas standalone application clients can be accessed offline (though they still need internet to send/retrieve email). Since almost all email servers and clients support POP3 and IMAP4, any client can be set up to retrieve email from any server. That’s why you can use your Outlook desktop app to access and manage your Gmail inbox.
Email servers, though at first glance might seem daunting, are actually very simple systems designed to deliver text messages (and now attachments) from point A to point B. The system’s core principles remain unchanged, but new technologies and solutions are being developed every day to allow greater reliability, security, and convenience.
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