Introduction

Before sending a large email campaign, it's common to test the message by sending it to a small group of recipients. This helps ensure the content looks right and that everything is set up correctly. However, these test messages often end up in the SPAM folder, which can be concerning. The reason for this is that receiving mail servers treat small test sends differently than large-scale campaigns. The way a message is handled depends on whether it's sent to a small group or to hundreds, thousands, or even millions of recipients. Different algorithms are used to determine how these messages are classified and where they end up in the recipient’s inbox.

Definitions

To understand why this happens, it’s important to differentiate between two key filtering processes: anti-spam filtering and greylisting.

Anti-spam filters are like bouncers at a club. They check the message's technical details to verify it isn’t SPAM. This includes ensuring the proper records (like SPF and DKIM) are in place to verify the sender. SPF records specify which servers are allowed to send on behalf of a domain, while DKIM provides a more complex form of verification tied to the sending server. These checks, combined with the Domain-based Message Authentication, Reporting & Conformance (DMARC) policy, help receiving servers confirm that the message is legitimate. In addition, anti-spam filters often scan the message content for suspicious links, images, or attachments that may raise red flags.

Once the anti-spam filters clear the message, it’s not necessarily ready to land in the recipient’s inbox. This is where greylisting comes into play.

Greylisting Explained

Greylisting is a separate algorithm used by many large email providers. After a message passes the anti-spam checks, greylisting determines where the message should go within the recipient's inbox (e.g., Inbox, Promotions, or Junk/Spam folder).

For small-scale sends, greylisting primarily considers individual recipient behavior. The receiving server uses the recipient's past interactions (e.g., marking messages as not SPAM) to determine where to place future emails. Over time, this profile updates, improving the accuracy of placement. This means that if recipients frequently interact with certain emails, those messages are more likely to land in their inbox in the future.

For large-scale sends, greylisting relies less on individual recipient behavior and more on patterns from previous interactions with similar messages. It examines the subject, content, and especially the sending domain across the server. For example, if Gmail users typically engage with emails from a shared sending domain, Gmail assumes that users will want to receive future emails from that same domain in their inbox. This is a key advantage of using a shared sending domain—by pooling senders with low volumes, we can improve deliverability without needing the extra setup required for custom domains, DNS records, feedback loops, and ongoing monitoring.