ISP Content Filtering

While ISPs still use IP blacklists to try to keep spam off of their systems, they increasingly use other tactics to keep their users’ inboxes clear of spam while still delivering requested messages.

One commonly used method is content filtering.

This tactic, while designed to help ISPs combat spam, can occasionally affect your permission-based email marketing campaigns too.

Fortunately, with a little understanding about how content filters work, you can reduce the likelihood of them affecting your messages.

What Do Content Filters Do?

Content filters “read” the subject and body of messages to determine whether they are likely to be spam.

When a message is received whose content meets the filters’ criteria for being labeled as spam, that message may be placed in the recipient’s bulk folder. In some cases, it can be bounced back to the sender as undeliverable.

Just like blacklisting, content filtering can result in false positives — email that the recipient has requested and wants, but is treated as if it were spam and not delivered.

Over my next two posts, I’m going to help you minimize your exposure to content filtering, and also give you some tips for dealing with it if it does happen to you.

Reducing Your Risk of Content Filtering

Being able to resolve a filtering issue is important, but it’s a reaction to something that has already occurred.

Before you find yourself in a position where you need to do that, get proactive! Take a few simple steps to minimize the likelihood of content filtering disrupting delivery of your messages.

Verify Your Subscribers.

When your subscribers add to your list, prompt them to confirm that they’ve provided you with the correct address and that the address owner did indeed request your information.

This does several things:

  • It keeps people from getting inadvertently or maliciously added to your list.

    This results in lower complaint rates. Since complaint rates factor into whether a message gets filtered, minimizing your complaint rate translates to minimizing your risk of getting filtered.

  • It keeps invalid email addresses off of your list, which reduces the volume and percentage of undeliverable messages that you send.

    Since undeliverable rates also factor into filtering rules, keeping invalid email addresses from being subscribed to your list will help you to avoid content filtering.

Follow Other Email Best Practices

There are numerous actions and concepts that you can utilize to minimize the likelihood of complaints and content filtering, such as:

  • Set subscriber expectations and build a relationship with them
  • Provide subscriber details in your messages (such as subscribe date, URL and IP address)
  • Send a plain text version along with your HTML messages
  • Thoroughly vet companies before including affiliate or other links to their sites in your messages

While these are certainly not the only steps you can take, they are the building blocks of a filter-friendly opt-in email campaign.

In my next post, I’ll discuss what to do when you do run into a content filtering issue.

UPDATE: the follow-up to this post, how to address false positives, is now available.