In this article, we look at how spam filers work and what can be done to ensure that our legitimate emails reach their target and aren’t wrongly filtered out as ‘spam’ .
Why We Need Spam Filters
Although we’re focusing on how to avoid spam filters, it’s worth noting how important they are to businesses. Figures vary between surveys but around half of email is known to be spam and more than 90 per cent of malware arrives in spam emails. For example, Gmail in 2020 recorded blocking more than 100 million phishing emails with its filtering system, and figures (Statista) from September 2020 show 88.88 billion spam emails were being sent worldwide every day.
It is therefore necessary to filter our emails to stop our email boxes from becoming filled with irrelevant and possibly dangerous emails such as phishing emails. Filtering out unwanted emails also makes it much easier to see our important emails. Bear in mind, mailbox providers have a commercial interest in wanting users to continue using their service and having an effective spam filter can help this happen.
How Spam Filters Work / Spam Filter Types
Spam filters vary in their design across mailbox providers, but there are broadly several types that use different signals and scores to judge an email as being spam (and direct them to your spam folder). For example:
Bayesian filters (and other heuristic filters) spot suspicious word patterns and frequencies in messages.
Blocklist filters block and remove emails from senders who are identified on a spammers list.
Content filters, as the name suggests, study the contents of an email with regards to language, such as words often used by spammers (special offer, discount) and inappropriate language. There are also ‘language filters’ but these are used to filter out messages with a different geographic language than that’s indicted by the recipient.
Header filters study an email’s legitimacy based on the characteristics of its header e.g., the IP address.
Rule-based filters apply rules established by users to incoming emails to decide whether they are delivered to the spam filter rather than the inbox. For example, these rules could be based on words or phrases in the message or header.
Other spam filtering judgements may be made using:
Engagement rates. For example, if a (sender) mailbox has a high number of emails that are sent, not looked-at at then deleted, this could indicate low engagement (a sign of spam) and lead to an email being filtered out.
Low mailbox activity. If an email box is rarely used apart from sending out large numbers of emails at once, this can be judged to indicate that it is a spam email account.
Identification and reputation (a reputation score signalling how trustworthy your emails are) are often the main reasons why emails land in the inbox or the spam folder, not just the email’s content.
Getting Your Emails Past Spam Filters
Most of us, however, are not spammers and have legitimate marketing, business, and personal messages, sent with good intentions that we need to ensure at least reach their target, hopefully to get read. Ways that emails can beat spam filters include:
Whitelisting : Since most major email providers (Google, Yahoo and Microsoft) automatically exempt whitelisted addresses from more scanning, ask known contacts to whitelist your email address in their spam filter, or to add your address to their contact list.
Use Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) and DMARC email authentication. For example, attaching DKIM signatures to an email (as an encrypted header), SPF records to link your emails back to the domain, and the DMARC protocol to protect your domain from unauthorised use e.g., spoofing are all (more technical) ways to indicate that your emails are not spam.
Where possible, avoid using spam trigger words in the header and content of an email e.g., buy, ‘double your’, XXX, earn, cash bonus, etc. There are many large lists of spam trigger words online and the guiding principles are to avoid anything that is sensationalising or over-promising.
Personalise emails e.g., with the recipient’s first name. This indicates that the email is less likely to be unsolicited.
Avoid using odd formatting (to stand out), strange use of punctuation or strangely formatted fonts. All of these are common signs of spam.
Keep your email deliverability rates high e.g., keep your email list clean (remove inactive users and invalid emails), make sure emails are compliant with current web laws, and add engaging text.
Only provide links to reputable websites.
Include an unsubscribe button/link in marketing emails.
Pay attention to spelling and grammar – use spelling/grammar checkers, and proof-read emails.
Make sure the ‘sent from’ name is easily recognisable e.g., your name and business name together.
What Does This Mean For Your Business?
The number of different factors that spam filters use to spot and isolate spam is, of course, good for us all, but can make it more challenging to design legitimate business emails that make it to their target. Paying attention to basic rules and checks (spelling, grammar, formatting, links, personalising, avoiding spam trigger words) and using a legitimate, well-maintained email account/platform with a clean list can provide a good basis for getting past spam filters. Looking into using SPF, DKIM and DMARC may also be worthwhile. It is important to get the best ROI in terms of time and money spent in creating and sending marketing and company communications emails and designing-in deliverability of emails is, therefore, vital.