How do spam filters work?

SPAM filters for incoming email go through a number of checks before routing to the inbox, the junk folder, quarantine or even returning them outright based on the total spam score. There are many filtering programs available, and each one has its own proprietary process for scanning email. Here is an example of how a filter may scan an email before letting it into the inbox.

1. Email arrives at server, SPAM filter is activated.
   • IP Blocks: Is the email coming from an IP on a blocked or black listed IP? (VR works hard to make sure our IPs maintain a good reputation.)
   • Viruses: Does the email contain a known virus, or anything that looks suspicious?
   • User Rules: Are there any words or images that the user has said are spammy? Conversely, has the user said emails from this sender are good and/or important? This is where asking your users to whitelist your sending address can help your email delivery immensely. 
   • Engagement and network history: Do other users across the network treat email from this sender as spam?
   • Intent: What does this email want users to do? Click a link? Download a file? How dangerous is that overall?
   • Spam Probability: Does this email look like other unrelated emails that the system knows are spam? Are there any common features, and how many?
   • Spam Score: How many spam features are in this email? Text formatting, keywords, and content features are assigned a point value, how many points you can have are based on how aggressive the filter settings are.
2. If the email checks out, the email is routed to the inbox. If the email is deemed too spammy, the email is routed to the spam box, or quarantined until more emails like this email come in and the system can re-evaluate. If the IP is blocked or black listed, or the spam score is very high, the email will likely be bounced back to the sender. 

The rules and limitations can change daily, so it's best to keep your email simple and relevant, and to focus on engaged customers to get the best return. If you want some more tips on how your design can improve your delivery, click here.