Their network, their rules

Much of the equipment and wires that the internet runs on is privately owned, nor is it a public utility in the traditional sense. The owners of the property have a lot of leeway to do what they like with that property. Yes, there are standards, but the standards are about interoperability. They describe things you have to do in order to exchange traffic with other entities. They do not dictate internal policies or processes.


picture of a globe with the word "strategy" written n red on it

As the owners of the equipment, companies have a lot of discretion about what they allow on their network, hence their network, their rules. As an example, both Twitter and Facebook are well within their rights to deny or allow traffic on their networks, no matter what the rest of us think about it. As they are not interoperating with other social networks, they make the rules.

This lack of interoperability extends to inbound email filtering as well. The filters can block any mail for any reason, and the sender has no real recourse. There are, of course, folks who can make changes to filters, but they are recipients, customers and business priorities of the filter maintainer.

Recipients are the final arbiters of what mail they want or don’t want. Many of the consumer mail filters are tuned to parse whether a mail is wanted or unwanted based on signals from the recipients. These aren’t the only signals used, mail has to be safe and come from a well behaved MTA. But most of the consumer ISPs care about keeping their users happy.

This is why so much advice, from myself and others, relies on getting the users to interact with the message. Most of the providers want users to be happy and so they will listen when users start complaining. Some providers, like Microsoft, even have formal processes to gather feedback from users on the accuracy of their email filters.

For business filters, customers are the primary driver. Most business filters, even those maintained by consumer ISPs, have an extra layer of filtering. This layer sits on top of the filters sent out to all customers, allowing each individual company to control their own incoming mail. Filtering priorities are set by the company.

Filters do what they’re told to do. Ultimately, business needs and priorities drive what filters do. The reason they can is because mail servers are private property and the owners can manage them the way they want to.

Your server. Your rules.

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Email filters

What makes the best email filter? There isn’t really a single answer to that question. Different people and different organizations have different tolerances for how false positives versus false negatives. For instance, we’re quite sensitive to false positives here, so we run extremely conservative filtering and don’t block very much at the MTA level. Other people I know are very sensitive to false negatives and run more aggressive filtering and block quite a bit of mail at the MTA level.
For the major ISPs, the people who plan, approve, design and monitor the filters usually want to maximize customer happiness. They want to deliver as much real mail as possible while blocking as much bad mail. Blocking real mail and letting through bad mail both result in unhappy customers and increase the ISP’s costs, either through customer churn or through support calls. And this is a process, filters are not static. ISPs roll out new filters all the time, sometimes they are an improvement and sometimes they’re not. When they’re not, they’re pulled out of production. This works both for positive filters like Return Path and negative filters like blocklists.
Then there is mail filtering that doesn’t have to do with spam. Business filters, for instance, often block non-business mail. Permission of the recipient often isn’t even a factor. Companies don’t often go out of their way to block personal mail, but if personal mail gets blocked (say the vacation plane ticket or the amazon receipt) they don’t often unblock it. But when you think about why a business provides email, it makes perfect sense. The business provides email to further its own business goals. Some personal usage is usually OK, but if someone notices and blocks personal email then it’s unlikely the business will unblock it, even if the employee opted in.
In the case of email filters, the free market does work. Different ISPs filter mail differently. Some people love Gmail’s filters. Other people think Hotmail has the best filtering. There are different standards for filtering, and that makes email stronger and more robust. Consumers have choices in their mail provider and spamfiltering.

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I’m sure by now readers have seen the article from Gmail “Spam does not bring us joy — ridding Gmail of 100 million more spam messages with TensorFlow.” If you haven’t seen it, go read it. It’s not often companies write about their filtering philosophy and what tools they’re using to manage incoming bad mail.

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The more I look at the industry, the more convinced I am that we’re in the middle of a fundamental shift in how email is filtered. This shift will change how we handle email deliverability and what tools we have and what information we can use as senders to address challenges to getting to the inbox.

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