Do Spam Filters Cause You To Lose Important Emails? Here’s The Truth

Spam emails are unwanted communications that find their way into your inbox. Some spam is simply irrelevant or a bit of a nuisance—like advertisements or newsletters that you haven’t subscribed to. These types of emails can affect end-users’ productivity, forcing them to scroll endlessly through their inbox each time they want to find something important. But spam can also contain malicious content, such as malware-riddled downloads or phishing links that direct users to spoofed websites designed to steal their credentials.

Spam filters are a popular means of dealing with both nuisance and malicious spam. They analyze all inbound (and, depending on the solution, sometimes outbound) emails and categorize them into groups such as spam, gray-mail, viruses, and phishing attacks. If the spam filter deems an email safe, it delivers it to the end-user as normal; if the email is malicious, the spam filter blocks it.

While spam filters are certainly an effective (and cost-effective!) way to improve employee productivity and protect your organization against email-based cyberthreats, some users report concerns around their spam filters blocking legitimate, important communications—which can disrupt business operations and cause more work for your IT team.

So, what can you do to make sure your spam filter only blocks unwanted emails? 

Before we can answer that, we need to take a look at how spam filtering tools actually work.

What Is Spam Filtering And How Does It Work?

Spam filtering tools use a combination of techniques to analyze inbound and outbound emails for indicators of unwanted or malicious content. While the specific combination of techniques used varies between solutions, they often include: 

Content Filtering

Content filters analyze the body of text within an email for use of language that’s often associated with spam, such as offering deals, discounts, or freebies, promoting inappropriate or explicit materials, or urging the recipient to respond quickly.

Header Filtering

Header filters analyze the header of an email to determine whether it’s being sent from an unwanted source. To achieve this, they check whether the email was sent from a known malicious IP address, and whether there are any indicators that the email was sent in bulk to a large number of recipients. 

Content Disarm And Reconstruction

Content Disarm and Reconstruction (CDR) tools break the email down into its various parts and strip it of any executable material, such as attachments and links, to make sure it’s safe. It then reconstructs the email and delivers it without those executables, so users can still read the message without interacting with any potentially malicious content.

Sandboxing

Sandboxing tools open emails, along with any attachments or links, in a secure environment isolated from your mail server. This allows them to assess the behavior of any executables in the email and decide whether they’re malicious or not.

Block Listing

Block list filters (also known as “deny lists” or “blacklists”) automatically quarantine or block emails from senders that have a history of sending unwanted or malicious emails. Some solutions offer a ready-to-go block list of known spammers, and some allow you to create your own custom block list.

Language Filtering

Language filters block emails that are written in languages in which the recipient isn’t fluent. 

Rule-Based Filtering

Rule-based filters enable you to create your own rules that determine whether the filter delivers, quarantines, or blocks emails. For example, you could create a rule that tells the filter to automatically block emails that contain certain words and phrases, or come from certain senders. 

Bayesian Filtering

Bayesian filters analyze the emails that you send to spam, examining their characteristics and content to learn which emails you’d prefer not to receive. They then use this information to set up relevant rules, which they apply to future inbound emails—helping to improve the filter’s accuracy. For example, if you consistently mark emails from a certain sender as spam, the filter will recognize that and automatically quarantine emails from that sender going forward. 

By combining several or even all of these techniques, spam filters minimize the likelihood of end-users interacting with malicious emails, and reduce the number of unwanted emails cluttering up end-users’ inboxes—which in turn enhances productivity. 

Do Spam Filters Block Important Emails? 

While many spam filters do a great job of blocking unwanted emails, they can also product false positives, in which they misidentify emails as spam. There are two main reasons for this: the first is that no security algorithm is perfect, and it’s almost impossible for any email security tool to produce absolutely zero false positives. 

The second reason, and the one responsible for the majority of false positives, is that spam filters aren’t a “set and forget” solution; you need to regularly tune them to make sure the filters and intelligence they’re using are capable of identifying the most recent spam techniques and senders. 

How To Correctly Configure Your Spam Filter 

Once you’ve deployed a spam filter, you need to regularly tune its configurations to make sure it’s blocking spam, phishing, and malware, but not hindering end-users’ productivity or business operations by blocking any important messages. 

Here’s our step-by-step walkthrough of how to do just that:

1. Start with a baseline configuration. 
   • Enable SPF, DKIM, and DMARC check to help verify that the sender is who they claim to be.
   • Set up reputation-based filtering to block known bad IP addresses and domains. Enable malware scanning for attachments and URLs.
   • Enable category-based filtering, which will block emails based on their content type (e.g., phishing, marketing, bulk mail) instead of simply marking them all as spam or graymail.
2. Fine-tune your spam scoring threshold. Most spam filters assign emails a “spam confidence level”, or SCL score. Instead of taking an “all-or-nothing” approach to this and outright rejecting all potential spam, you should use layered handling, which delivers, quarantines, or rejects emails based on certainty:
   • Set high-confidence spam to be quarantined or blocked.
   • Set medium suspicion emails to be quarantined or sent to junk. Set low suspicion emails to be delivered with a warning banner.
   • Send end-users daily or weekly quarantine digests so they can check for false positives.
3. Maintain allow lists/ safe lists. 
   • Add trusted and authenticated email addresses and domains to a global allow/safe list. You can then either set a higher SCL for these senders, or enable them to bypass the SCL entirely. 
   • You can also teach end-users how to mark senders as safe themselves within their email client. However, make sure that any senders being marked as safe are using proper authentication!
4. Continuously monitor and adjust. Regularly review your logs and quarantine reports, so you can: 
   • Identify recurring false positives. 
   • Adjust your SCL thresholds and anti-spam rules based on actual traffic patterns. 
5. Train your end-users on how to:
   • Check their quarantine digest.
   • Request the release of any important messages that were sent to quarantine mistakenly.
   • Report suspicious emails that haven’t been successfully blocked by the spam filter. 

By following these steps, you can be confident that your spam filter is saving your end-users from overly-cluttered inboxes and helping to protect them from interacting with any malicious emails—whilst making sure they’re still receiving all the important messages they need.

Learn more about the best email spam filtering solutions.