All filters are not equal


Many questions about delivery problems often assume that there is one standard email filter and the rules are the same across all of them. Unfortunately, this isn’t really the case.

The biggest divide is consumer versus business filters. Business filters don’t really care about things like engagement. A sender could have near perfect engagement with a message to a business. But a decision maker inside the company can still decide that mail doesn’t get in. There’s no appealing to permission or wanted mail. Employee mail is provided for the good of the business, not for the good of the individual user or the sender.

There are other less obvious divides between filters as well.

I frequently refer to “webmail providers” (Oath, Microsoft, Gmail). These are companies that control the mail delivery and, for the bulk of their customers, control the mail client as well. They can use engagement filters because they have more data. Other companies, like broadband providers or web hosting services, don’t have the same level of access to customer behaviour, so they can’t heavily use engagement as part of their filtering processes. They may have some access to IMAP folders, depending on their setup, so they can look at some engagement flags.

Filtering companies also have their own type of filters. In many cases, though, they have no access to any engagement filters. They handle mail at a discrete point that starts during SMTP sessions and ends when the mail is handed off to the local delivery agent. These companies cannot use engagement as part of their filtering process all, they simply don’t have access to that data.

Understanding what data filters act on and what data they have access to can inform how to deal with blocks and delivery problems.


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  • Hi Laura,

    Thanks for the excellent post. I was wondering you had any insight as to whether the same best practices for Gmail email addresses can be applied to businesses using Google Apps?

  • It’s the same, but not the same.

    I believe Google Apps / G Suite filters use the same basic filter engine that the consumer filters do. But, some of the assumptions and tolerances of what users expect and accept is different. For instance, businesses often have less tolerance for false positives – that is they are more sensitive if a business email goes to bulk/spam instead of the inbox. Those kinds of differences mean the filters are close, but not identical.

  • Hi Laura,

    The better enterprise-level mail filters (some of the best ones are open-source), will auto-whitelist addresses found in outgoing mail. So if a user emails the list, the list can be whitelisted for either that user or for the entire domain, depending upon how things are configured. This of course doesn’t help much if the list never receives email from subscribers but having the user reply to an email instead of clicking a link could potentially help in this case.

    Using client and IMAP access data to measure engagement for purposes of reducing filtering false-positives is a great idea though!

    – Phil

By laura

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