Cracking the Inbox Code: Google

Cracking the Inbox Code: Google

It can be challenging to consistently achieve inbox delivery at Gmail and Google Apps. One reason is that Gmail has very complicated filtering strategy, which learns from and classifies hundreds of items within a message to detect and prevent spam. They are machine learning-based and measure hundreds of things about incoming mail. The filters are continually adapting to spam threats and updating how they treat specific mail.

The impact of this approach is that filtering is far from static. What passes to the inbox now, may fail in short order.

We’ve put together a bit of intel about Gmail filters to support senders in ensuring their programs are in tip-top shape to achieve inbox placement at Gmail/GoogleApps.

Google’s spam filters

  • Google’s leverages a content scan to determine if message is bulk/mass marketing.
  • The filters also recognize when an email is attempting to collect sensitive recipient information. This will trigger “potential phishing message” notification and will also flag the mail as spam.
  • Gmail spam filtering is very user feedback-driven. Subscribers train the filter when they click on “This is Spam” or “This is Not Spam.”
    • Low complaints and high “this is not spam” rates = positive engagement
  • Adapts to spam threats through machine learning and constantly updates filtering protocol.
  • GoogleApps Spam Filter: In 2007, Google acquired Postini a security company focused on email. In 2012 they started shifting customers away from the Postini anti-spam filter and towards GoogleApps.
  • Further, GoogleApps hosted email solution for businesses has rapidly earned marketshare, so most B2B senders are significantly impacted by delivery failures to this domain.
  • On the bright side, Google rarely rejects mail outright (though throttling is common during IP warming). Most often, Google will deliver questionable mail to the spam folder. The challenge is that it’s fairly difficult to navigate to the spam folder and with priority inbox, there are even more mechanisms to sort through mail, which may negate the subscribers willingness to search for mail in their spam folders.


  • Follow best practices for opt-in (confirmed opt-in is always best).
  • Set clear expectations regarding the type and frequency of the email you will send and honor the commitment.


  • Authenticate using SPF and DKIM (strongly encouraged) and publish your practices with DMARC.
  • Gmail considers transactional mail to be the most important mail they deliver. Therefore, you should us separate IPs, subdomains, and sending addresses that are used for other email streams. Don’t combine with promotional mail.
  • Set up a branded envelope from. (Available through Marketo, contact your CSM.)
  • Employ the list unsubscribe header. Think of it as a poor man’s feedback loop at Gmail. If you have a strong reputation at Gmail, this functions as a mechanism to return an unsubscribe request to you when your subscribers mark your message as spam. (Marketo uses the List Unsubscribe Header)


  • Ensure Your Links Aren't on a Blacklist or Inactive If your message is marked as suspected phishing mail, check to see if any of your URIs are on URL/domain based blacklists like URIBL, SURBL and SpamHaus DBL. Ensure all URLs within your content are active and resolve as expected.
  • The presence of blacklisted and/or inactive URLs in your content will negatively affect your reputation with GoogleApps and your inbox placement. This includes third-party URLs like etc.
  • Manage engagement – monitor recipient activity and re-engage or remove inactives. Contacts who never open your messages could become a liability for you if they start to file abuse complaints , which would ultimately impact your active recipients too. As a general rule, contacts with no activity for more than six months should be removed from your list or moved into a special “reactivation” messaging track designed to try to re-engage them one last time before dropping them from your list.
  • Encourage engagement.
    • Ask your recipients to mark your messages as “not spam.” This will alert Gmail that your recipients want to receive your messages, help your overall engagement with Gmail and improve delivery to the inbox.
  • Invite contacts to add your sending “From” address to their Gmail address book.
    • Ask your recipients to click Gmail’s yellow priority inbox icon and have them “mark as important.” This will ensure your mailings take the highest priority in your recipient’s inbox and are always listed as “important.”


  • Relevance! Send only targeted, relevant email to keep subscribers engaged.
  • Implement behavioral marketing best practices to increase message relevance and boost opens and clicks. Sending emails triggered by a person’s behaviors, preferences or demographics is one of the best ways to ensure a positive customer experience and increase recipient interaction with your messages.
  • Subject Lines: Keywords and symbols in your subject line can trigger the GoogleApps filter. Avoid the use of the prefix “ADV:” (for advertisement), dollar signs or multiple exclamation points.
  • Avoid Certain Number Patterns: GoogleApps has a filtering function that matches number patterns and/or sequences that look like credit card or Social Security numbers. They consider this an indication that the mail is a potential phishing message and it will result in a block of your messages. The following are examples of the patterns that will trigger the filter:
    • 16 digit number patterns (credit card):
      • n nnnn nnnn nnnn nnnn
      • nnn-nnnn-nnnn-nnnn
      • nnnn nnnn-nnnn nnnn
      • nnnnnnnnnnnnnnnn
    • 9 digit number sequences (Social Security) – these digits can be separated by spaces dashed or periods:
      • nnn-nn-nnnn
      • nnn nn nnnn
      • nnn.nn.nnnn
      • nnn-nn nnnn
  • Remind recipients to click on the “Always display images from this address” button. This enables your recipients to see your HMTL emails as they were intended and also goes toward positive recipient engagement – especially important given how critical your images are to your message.
  • Create Gmail-specific campaigns with subject lines and a targeted call to action for Gmail users.
  • Avoid URL shorteners. As noted earlier, URL reputations have a significant weight on Google’s filtering, and URL shorteners have been abused by spammers to deliver malicious email, such that many of the URL shortener domains are permanently blacklisted or blocked.

Additional Resources

Gmail’s Bulk Sender Guidelines

Troubleshooting for bulk email senders

Bulk Sender's Contact Form

The Learning Behind Gmail's Priority Inbox

Why am I seeing "via" in the From line?

Google Product Forum - Mail server blocked by gmail as a bulk sender

15 Email Deliverability Best Practices for Gmail

Special thank you to Carmi Lopez-Jones for helping draft this blog post!

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