What is a spam filter?

A spam filter is software that checks every email sent to your address and decides whether it looks like spam. Emails that pass go to your inbox. Emails that look like junk mail or a scam usually go to your spam folder instead, so you only see them if you go looking. Spam can promise anything from a new car to a cheap loan.

How does spam filtering work?

Many spam filters work with a score. The filter runs a series of tests on each email, and every sign of spam adds to the email's spam score. When the score goes over a set threshold, the filter marks the email as spam and moves it out of your inbox.

The tests look at different parts of an email:

• The sender: a filter is more likely to flag an email from a domain with a bad reputation, or one that has sent spam before. Many filters also check the sending server against blocklists of known spam sources.
• The headers: from the details of who sent the email and how it travelled, a filter can check whether the email really comes from the domain it claims. It can do this with three records the sending domain can have, called SPF, DKIM and DMARC.
• The text: words and phrases that are common in spam, such as "act now" or "limited time offer", and tricks like a subject line in capitals or a row of exclamation marks.
• Links and attachments: a link to a phishing website or an attachment with malware makes a filter more likely to catch an email.
• What other people reported: some filters also learn from what their users report as spam, which helps them catch similar emails.

In most cases, no single test decides on its own. What counts is the total score.

Bayesian filtering: how a spam filter learns

Bayesian filtering is a way for a spam filter to learn from the email it sees. It looks at the words and short pieces of text in each email. For each one, it works out how often it turns up in spam and how often in legitimate email. From those odds, it calculates how likely it is that a new email is spam.

The filter gets better the more examples it sees. An email that is marked as spam, or moved back out of the spam folder, can teach it something about your mail.

Spammers try to work round this. Their software breaks up and misspells words so the filter doesn't recognise them. You might see "1nstead", with the digit 1 in place of the I. Sender addresses get a similar trick, such as a zero in place of the letter O. Gmail, for example, warns you when an address looks very similar to the address of a sender you know.

What happens to the emails a spam filter catches

Most of the time, an email the filter catches goes to your spam folder, sometimes called the junk folder. An email that contains a known virus or other harmful content may be deleted straight away instead, so it never reaches you at all.

Spam filtering isn't perfect, and filters sometimes get it wrong. A legitimate email can end up in your spam folder because it trips enough tests, for example a newsletter full of offers and links. This is called a false positive. Check your spam folder now and then, so you don't miss an order confirmation or a reply from a customer. The opposite also happens: a spam email gets through to your inbox.

Why a spam filter matters

Some scam emails try to get you to click a link to a website that steals your passwords or bank details. This is called phishing. Others link to a website that may download a virus or other malware to your computer.

Even if you're good at spotting spam yourself, a cleverly written scam email is easy to mistake for a real one. Your business inbox needs a spam filter, which checks every email before you get the chance to open the wrong one. If you're not sure your email has one, check your email settings or ask your provider. Scam emails are only one of the email threats to watch for.

How to help your spam filter work well

A few habits help your filter catch more spam and fewer real emails.

• When a spam email gets through, report it or move it to the spam folder instead of only deleting it. That helps the filter learn to catch similar emails next time.
• When you find a legitimate email in the spam folder, mark it as not spam. Future emails from that sender are then less likely to end up there.
• Add senders you trust to your contacts or a trusted senders list, which many email services let you do. In our webmail, emails from a trusted sender skip the spam filter and always reach your inbox, so keep the list to addresses you know and expect email from.
• Block a sender who keeps getting through, if your email service lets you. With us, emails from a blocked sender go straight to your spam folder.
• Don't reply to a spam email or click its unsubscribe link, as that tells the spammer your address is active and usually brings more spam. A newsletter you signed up for yourself is different, and you can unsubscribe from it as normal.
• In the UK, you can report a scam email to the National Cyber Security Centre (NCSC) by forwarding it to the address on its reporting page. You don't need to forward the ones already in your spam folder.

To get fewer spam emails in the first place, be careful where you share your email address. For example, avoid putting it in full on public web pages or forums.

Our virus and spam filter

With us, all your email accounts come with a virus and spam filter, which we switch on by default when you create an account. It checks every email delivered to your account and moves spam to the folder called Spam. When an email contains a known virus or other harmful content, we delete it. We update the filters automatically every hour, so they keep up with new viruses and spam.

In webmail, where you read your email in the browser, you can manage both lists: the senders you trust and the senders you've blocked.

Every mailbox in our email plans is protected from spam and viruses, and comes with webmail and a calendar.

Questions about spam filters

Why do my own emails end up in other people's spam folders?

The recipient's spam filter runs the same tests on your email as it does on any other. A sending domain with a poor reputation, text full of sales phrases, or a domain without SPF, DKIM and DMARC records can all push your email over the threshold. If your email is with us and your domain uses our name servers (the addresses that tell the internet where your domain's settings are kept), DKIM is on automatically, and new domains also get our standard SPF and DMARC records. On an older domain, you can switch SPF and DMARC on in your DNS settings if they're missing. If your domain uses another host's name servers, even one registered with us, you add the records at that host. You can still add DKIM there, and our support can help you work out which records your domain needs. These records are part of good email security, because they let the receiving server check that your emails really come from your domain.

Can I turn the spam filter off?

Yes. With our email, you can switch the spam filter off for an account in the control panel or in webmail. We recommend you keep it on for all your email accounts, so you stay protected.

Get email with a spam filter built in

For your business email, choose an email plan with us and set up your first mailbox: the virus and spam filter is already working when the first email arrives.