The Key to Inbox Placement

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Globally, hundreds of billions of emails are sent every day, and many estimates consider around half of those to be spam.  Inbox providers like Gmail and Microsoft face the problematic task of filtering out all that junk while still delivering as much legitimate email as possible.  How do they decide if your email should be delivered to the inbox, the junk folder, or blocked altogether? Instead of relying solely on checking email authentication and content for every message, inbox providers also use the concept of sender reputation, which is a measure of your trustworthiness as a sender.  For example, if you’ve sent too much unwanted mail every day for the past month, they’re willing to bet that you’ll do the same today and they will block or junk your mail, even if it passes their other filters.  

You might be thinking, “That’s fine, I don’t send unwanted mail”. But in this case, the inbox providers get to decide what’s unwanted and what’s too much. Their definition of “unwanted” isn’t limited to prototypical spam either - many marketers send unwanted mail without even knowing it.  So assuming your email authentication and content checks out, sender reputation is the largest remaining factor for inbox placement, and managing your reputation becomes critical to realizing the potential of your email marketing strategy.

A Credit Score for Email Quality

The first step to managing your sender reputation is understanding how inbox providers track it. You can think of sender reputation like a credit score based on your history of email quality.  Much like a credit score, sender reputation is harder to repair than to maintain: you can’t get points for sending high quality mail if your reputation prevents you from inboxing.  Additionally, a lack of sending history means you won’t have a sender reputation. Inbox providers will accept some mail from you, but you’ll have a low limit until you gain their trust. You can build a positive reputation by sending engaging, properly authenticated mail at consistent volumes. Your reputation will be negatively impacted by behavior generally associated with spam or indications that your email was unwanted, including:

  • Spam Complaints - When a recipient clicks the “Spam” or “Junk” button in their email client
  • Hard Bounces - Invalid addresses that do not exist or have expired
  • Spam Traps - Email addresses specifically designed to catch spammers in the act. All email sent to spam trap addresses is considered spam. There are three major varieties of spam traps:
  1.      Pristine - A new address that is created but never opted-in for any email
  2.      Recycled - An inactive, old address that has expired and was revived as a spam trap
  3.      Typo - Addresses at misspelled domains (e.g. or
  • Low Engagement - If recipients don’t interact with your message, commonly measured on the sending side by opens and clicks
  • Content Blocks - Messages blocked due to spam keywords or link reputation
  • Inconsistent Volume - Sudden volume spikes or too much mail at once
  • Technical Issues - Unauthenticated mail or other infrastructure issues

Unlike a credit score, inbox providers keep their own records of your mail quality by sending domain, IP address, or any combination of identifying elements, and it varies from one provider to the next. There’s no central repository for sender reputation, and no singular place to get a report on your reputation.  Sender reputation is also augmented by third party blacklisting services like Spamhaus, but this typically only works against you. If you get blacklisted on a third party service, you could have a poor reputation at an inbox provider even if you’ve never sent email to them.

Difficulty in Measurement

So how do you gauge your sender reputation? You can get a partial picture by looking at the data you have access to: Your sending volume, complaints, hard bounces, content blocks, and opens/clicks. There are also sites like where you can check multiple blacklisting services all at once.  Issues in any of these areas should definitely be addressed, but these aren’t perfect indicators as you could still have a reputation problem even if you don’t see the symptoms. Not all inbox providers will share spam complaint data, any spam trap worth its salt is kept secret and indistinguishable from a normal address, and engagement can vary widely depending on your industry and marketing strategy, so data on the sender side is limited.  Furthermore, continued progression in privacy policy trends will only make it more difficult to get data from the place where your reputation is truly measured - the recipient side.

Practical Management

Given that measuring and reacting to negative symptoms isn’t enough, the best way to manage your sender reputation is to avoid these issues altogether by eliminating the possibilities for them to exist. Spam complaints, hard bounces and spam traps all stem from known, and sometimes common, lead generation and sending behaviors like sending without permission, or sending to recipients for years even though they’ve never engaged. You can easily preempt reputation damage by implementing industry standard best practices in your marketing operations to ensure you only send email that your recipients both want and expect.  By focusing on delivering high quality, engaging email to interested recipients, you’ll also see increased effectiveness and a better return on your marketing automation investment.