A Beginners Guide to ISP Inbox Delivery


 A Beginners Guide to ISP Inbox Delivery

As we all know, email is a big part of our daily lives. It is also the last bastion that ISPs have in terms of their marketing departments. In order to help users better manage their inboxes and work with emails more efficiently, there are a number of tools available including mail sorting apps, spam filters, and data analytics software. However most people who use these might not know how they are put together or how they work on an everyday basis.

In this blog post I will be walking you through some basics about what happens when your email arrives at your ISP inbox and explaining things like what email servers do and how ISPs create features like spam filters. We will also be looking at how your ISP handles (or can handle) things like unsubscribes, bounced email addresses, and personalised options.

Let's start off with a quick primer on the basics by looking at the steps that take place from when an email is sent all the way to when it arrives at your inbox.

In order for an email to be delivered you must trust the sender. It means that they have sent it from their server and you have not otherwise independently verified that they are in possession of the email address they say they own. When you click on a link in an email or reply to one someone has sent you your browser requests that domain for information about where to send its response (headers). It is something like 'http://example.com/page.html' and the server will always respond with those headers. When your browser receives that response it will then send a request to your ISP email server to see if you have email associated with that domain for you. That email server does a DNS lookup on that domain and finds the IP address of the mail server associated with that domain. It then sends a message (SMTP) to this mail server saying 'Please deliver this message for me'. This is done over port 25 on TCP/UDP and not through SMTP itself since there is no authentication going on. The mail server validates the headers through DNS and checks its own configuration to see if you have a mailbox on that server. If not, then the email is bounced back (see below). However if your email address is present then it will accept this message and store it in a mailbox associated with your name and domain (because that is the job of an SMTP server as we will see later). The mail server will usually store up to 1000 messages in this folder for you before deleting them. This is called your 'Inbox'.

However, sometimes ISPs want more control over how their users interact with their mailboxes, so they add features like filters which can move some or all of your incoming messages into other folders. This is a job best done by another piece of software which will be called the 'Mail Message Processor'. This was first seen in Netscape 2.0 which came with the product now known as Mozilla. It is also the first piece of software that you need to get to know if you want to better manage your email at hand.

On top of being able to 'push' messages into other folders, ISPs have access to more features which can help clean up their customers' inboxes. An ISP can add an unsubscribe link onto every email and also supply a bounce address that is used when an address bounces off their system (see below).


As you can see, each service that is involved in sending and receiving emails has a specific role to play. Email servers play the vital role of making it possible for you to send and receive messages with other people who use email, while mail message processors offer more features to help you filter your incoming messages. Inbox delivery is a complex process that depends on multiple layers of software working together. It's important to know how all of this interacts because it will help you manage your inbox in a better way by understanding how things work, as well as knowing what other tools are available for you to use and why they are necessary.

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