Email Filtering and How It Stops Spam

What Is Email Filtering?

Email filtering is the basic process of sorting out which messages to let through, block, or put in the quarantine based on what it knows is good or bad. A spam filter is a more advanced type of email filter that specifically looks for junk mail (spam) and keeps it separate from genuine emails.

If you didn't have spam filters, your inbox would be wide open to all sorts of threats including:

• Phishing scams: Emails that try to trick you into handing over personal details by pretending to be from a real organisation.
• Spear phishing: Targeted phishing attempts that use your personal details to try and make it look like the email is really from someone you know.
• Spreading viruses through email: Files sent to you in emails that can infect your computer if you open them.
• Stealing your identity: Emails that are all about stealing your personal details for some dodgy reason.

And to make matters worse, without a decent spam blocker your inbox would quickly become a nightmare to manage – a never-ending stream of unwanted marketing emails, newsletters, and other promotions.

In business, email filtering does its job in both directions – filtering in as well as sending out:

• Inbound filtering checks every message coming in to make sure it's not malicious or just plain rubbish.
• Outbound filtering does the same when it comes to emails going out. It stops anyone accidentally sending sensitive stuff to the wrong people, or worse, getting hijacked and sending spam to everyone on the contact list.

Both types use lots of different checks, from making sure the sender is okay to using clever software to work out if the email is genuine or not, to keep email safe and secure.

How Email Filtering Works

Modern email spam filters are like layers of an onion, working together to identify and block spam – each one checks for different signs of trouble:

Layer 1 (Check the Sender's Bad Reputation): Most email spam filters start by checking if the sender's email address is from a known troublemaker. They check to see if it's on a blacklist (a list of IP addresses and email domains that have sent spam before). If it is, they block the whole thing right off the bat, so the spammer can't even get the message to the spam folder. Of course, each email provider has its own list of blacklisted addresses – it's not a one-size-fits-all system.

Layer 2 (Look at What's Inside the Message): Once a message has passed the first test, the spam filter starts to look at what's inside the email itself. They're looking for words and phrases that sound a bit too good to be true – things like "you've won the lottery" or "limited time offer". They also take a look at things like formatting and links in the email. If the link is to a dodgy website or an attachment looks suspicious, it might get flagged as spam. Even if a spammer is trying to get around the first layer by using a new IP address, the second layer will often catch them.

Layer 3 (Looking Under the Hood): Spam filters also check the behind-the-scenes stuff - things like who the message was sent to, how often it's being sent, and who is sending it. They're trying to spot patterns that might suggest a spammer is at work.

Layer 4 (Authentication Protocols): Since email was first invented, a few authentication systems have been developed to help stop spammers in their tracks. SPF (Sender Policy Framework) is one, it checks to see if the email server is allowed to send email on behalf of a particular domain. DKIM (DomainKeys Identified Mail) adds a signature to the email so that you know it wasn't tampered with on the way. DMARC (Domain-based Message Authentication, Reporting, and Conformance) ties it all together and tells receiving email servers what to do if these checks fail.

Layer 5 (Spam Filters Get Smarter): Some of the newest email spam filters use a bit of artificial intelligence to try and figure out which messages are spam. They look at lots and lots of messages and try to spot patterns that might indicate spam. And the best bit is that these systems get better all the time – every time you mark a message as spam or move it out of the spam folder, you're helping to teach the system to be better at spotting spam in the future.

This multi-step filtering process often gets a boost from sandboxing, which allows email attachments to be opened in a safe, isolated place. If the email does something dodgy, it gets quarantined before you even get a chance to look at it.

Types of Email Filtering Techniques

With multiple technologies coming together to make spam detection and data protection more accurate, here are the most common ones you'll see:

Filtering Options in Popular Email Services

Spam filters work in a few different ways, and your ISP's filter is just the start. That's because it's already blocking some dodgy messages before they even reach your email provider, which has its own filtering systems to sort through.

For big businesses and organisations, there's a hosted spam filter service that offers extra, dedicated protection – managed by security pros, not some script. Home users, on the other hand, can beef up their email security by using a mail client app that has its own built-in filters.

Most big-name email providers, like Gmail, Outlook, and Yahoo Mail, all come with some basic filtering options built right in. Users can create their own rules, block unwanted senders, or even redirect newsletters to separate folders – it's all pretty straightforward.

Want to give it a go? These are the guides on how to set up filters for the popular email services:

• Gmail email filters
• Outlook email filters
• Yahoo Mail filters
• Hotmail filters
• iPhone email filter
• Android email spam filter

The Benefits of Email Filtering - Why You Need It

Getting your email filtering strategy right can make all the difference to your inbox and your sanity - here are just a few of the benefits:

• Keeps your account safe from phishing scams, business email compromise, ransomware and identity thieves trying to get in on the action.
• Cuts down on spam and means you get to spend more of your time and energy on the things that matter.
• Helps keep you on the right side of the law by ensuring you are complying with things like the GDPR and HIPAA.
• Keeps your domain name looking good by stopping spam from going out in the first place.
• Helps automate some of those tedious tasks that are normally a pain in the neck.

How to Get Your Email Filtering Strategy Right

For the IT teams and organisations out there:

• You need to get the right balance between blocking and allowing to avoid false positives.
• Make sure your block and allow lists are always up to date.
• Integrate your filters with your Security Information and Event Management (or SIEM for short) tools so you can keep an eye on what's happening.
• Keep your AI models training by feeding them user feedback all the time.
• Don't forget, technology is not the only answer - employee awareness training is also really important here.

What Can Users Do to Improve Spam Filtering?

Even the most sophisticated spam filters with machine learning algorithms need your help to reach their full potential. Here are several essential practices to improve your spam protection, along with several behaviors you should definitely avoid because they would surely invite more spam into your inbox.

✅ DO: Mark messages as spam using your email client

When you identify a spam message that slipped through your filters, don't just delete it—mark it as spam. By doing so, you provide valuable training data to your email provider's spam identification systems.

How to mark emails as spam in popular clients:

• Gmail: Select the message, then click the "Report spam" button (icon that looks like an exclamation mark in an octagon).

• Outlook: Right-click the message, select "Report," then "Report junk" or "Report phishing."

• Yahoo Mail: Select the message, then click the "Spam" button in the toolbar.

❌ DON'T: Reply to spam messages

Never respond to unwanted messages, even if it's to request removal from a list because spammers use replies to confirm that your email address is active. When that happens, you can be sure that more spam will follow promptly.

⚠️ That's why clicking "unsubscribe" links in obvious spam (as opposed to legitimate marketing emails) is a bad practice.

If you're unsure whether an unsubscribe link is legitimate, or if you simply want to unsubscribe more efficiently, Clean Email's Unsubscriber feature offers a safer alternative. This tool attempts to send proper unsubscribe requests on your behalf and, if that doesn't work, automatically blocks the sender directly. This two-pronged approach guarantees you'll stop receiving messages whether the sender is legitimate or not, all without risking your email security by clicking suspicious links.

✅ DO: Keep your inbox organized

A cluttered inbox makes it easier to miss suspicious messages or accidentally engage with spam. Clean Email's Smart Folders, Cleaning Suggestions, and Auto Clean features can significantly improve your ability to quickly identify potential threats that may have bypassed your spam filters for email.

Smart Folders automatically categorize your messages into intuitive groups like Social notifications, Finance and Insurance, Online shopping, or Messages from "no reply" to make it simple to review similar emails together and spot anything unusual.

The Cleaning Suggestions feature analyzes your email patterns and recommends actions based on your (and other users') habits and best practices.

Auto Clean takes organization to the next level by applying your chosen rules automatically as new emails arrive to save you time.

Clean Email also offers powerful sorting and grouping options in the inbox view that give you complete control over how you visualize your messages. You can easily see, for example, only large messages grouped by sender with the newest on top.

These flexible organization tools make filtering emails effectively much simpler and help prevent spam from hiding among legitimate messages.

❌ DON'T: Click links in suspicious emails

Even previewing suspicious messages can sometimes be risky, but clicking links within them is particularly dangerous because they may lead to phishing sites designed to steal your credentials, spoof your email address and send spam from your account, or download malware to your device.

Before interacting with any link, hover your cursor over it (without clicking) to reveal the actual destination URL in a small popup or in your mail client's status bar. This simple action can often reveal deceptive links that appear legitimate at first glance but actually point to suspicious domains (such as various URL shorteners or slight misspellings of familiar websites).

Some mail clients now offer built-in link protection features for added security. Microsoft Outlook's Safe Links technology, for example, checks URLs in real-time when you click them, scanning for malicious content and warning you before allowing access. You can learn more about Microsoft's Safe Links feature to better understand how it protects you from dangerous content.

✅ DO: Block problematic senders and domains

When you receive spam from particular senders or domains, blocking them prevents future messages from reaching your inbox. Clean Email's tools make this process effortless by allowing you to block entire domains with just a few clicks:

1. Start by clicking the Senders option in the left-hand navigation menu to view a comprehensive list of everyone who has emailed you.
2. Select the problematic sender from the center pane and click the Block button in the right-hand panel.
3. A confirmation dialog will appear where you can choose whether to move existing messages to Trash (recommended for spam senders).
4. Click Confirm, and you'll never see messages from that sender again.

One advantage of using Clean Email for blocking is that it works consistently across platforms—the same steps can be used whether you're trying to stop junk mail on iPhone or desktop. This cross-platform consistency is especially valuable if you access your email from multiple devices.

❌ DON'T: Share your email address publicly

Avoid posting your email address on public websites, forums, or social media where it can be easily harvested by spammers' automated tools. If you must share your address online, consider using alternatives:

• Write it in a format that humans can understand but bots can't easily parse (e.g., "username AT domain DOT com").
• Use a disposable email address for one-time registrations.
• Create separate email addresses for different purposes (personal, shopping, etc.).

Gmail users can take advantage of a clever built-in aliasing feature by adding a plus sign (+) followed by any text before the @ symbol.

After creating these aliases, you can use Clean Email's Auto Clean feature to automatically process these messages according to their source. For instance, you might create a rule that automatically deletes all messages sent to your "shopping" alias after a certain period or moves newsletters to a dedicated folder.

✅ DO: Block images in HTML messages

Many spam messages contain invisible tracking pixels or "web beacons" that notify the sender when you open an email. By default, most email clients now block remote images unless you specifically allow them. Keep this setting enabled to prevent spammers from confirming that your address is active.

In Gmail, you can control image loading by going to Settings (gear icon) → See all settings. Scroll down to the "Images" section and select "Ask before displaying external images" rather than "Always display external images."

Once saved, Gmail will no longer automatically load images in your emails. When you receive a message with images, you'll see a "Display images below" option that you can click only for trusted senders.

The Take Away

The bottom line is this - for organisations and individuals alike, getting your email filtering right, combined with a bit of common sense, is the key to keeping your inbox safe and secure. Clean Email can help by automating some of the tasks that make your email less enjoyable and more of a hassle.