What spam filters think when they see your email

Designing and building a lovely email template takes time, effort, imagination and patience – but what happens if all that effort goes to waste purely because the email got caught in the spam filter?

If you want your emails to be read by the recipient (don’t we all?), then it’s best to avoid doing something ‘spammy’ which will start those spam filter alarm bells ringing.

What are spam filters?

For those of you that don’t know, when you send an email there are barriers it must cross before it arrives in the desired inbox.

The message must get past the ISP spam filter, and then the recipient’s email client spam filters. Every email client’s spam filter works in a slightly different way, but we’ll try to explain their general ‘thoughts’ (NB: they don’t have brains).

How spam filters think

Spam filters go through the following ‘thought’ processes, before either allowing or blocking your email:

Subject line: Did the sender user any spammy words or phrases?

The subject line is one of the main reasons that your email would be flagged as spam. And if the email doesn’t get flagged as spam, it will still be judged by your recipients. This will determine if they open and read the email, so it has to be relevant.

A few subject line tips:

-    Get the point across quickly. You don’t have much time before the recipient moves on.

-     Make it intriguing and entice the recipient to open the email.

-     Avoid spammy words such as, free, limited time, money off, click now etc.

-     Control your use of punctuation!!!!!!!!!!!!!

-     Avoid swearing and expletives.

-     Don’t try to fool the spam filters by replacing numbers with words, e.g. H3LLO.

The recipient: Did the sender know the recipient’s name? Or is it just addressed to a basic email which they could’ve guessed?

Hands up if you’ve ever received an email entitled ‘Dear valued customer’? Sorry to break it to you, but you’re not really that valued are you? If you were, the company would’ve taken extra steps to find out your name.

This is the same way that spam filters think about generically titled emails. It’s obvious that a computer created the campaign and not a person. Spam filters appreciate personalisation over generic emails.

The content: Is the HTML programmed correctly? Has the sender included a plain text version? Does the content contain spammy words or phrases?

Spam filters look for clues everywhere:

-     Avoid spammy words in your email content. ‘Click now to download this FREE ebook!!’ may not be spammy on purpose, but it can flag some filters.

-     Don’t send just a single large image. Spam can be hidden within images, so find a balance between images and text.

-     Always include a plain text version. Spammers tend to be lazy and don’t include plain text versions. It may seem like another step and another task to complete, but the majority of systems will automatically generate a version for you.

-     Look out for link shortening domain issues. Some links can include domains that have been blacklisted.

-     Make sure your HTML is correct. Broken images, missing tags, and non web-safe colours can also trigger spam filters.


The sender’s IP address: Has this email been sent from a server that is on a blacklist or is known to be spam friendly?

Some spam filters will sync with blacklists, and if you’re on a blacklist, the spam filter may not let you through.

Unfortunately, sometimes innocent senders can end up blacklisted, even if they aren’t sending spam messages. For example, if a spammer is sending unsolicited emails and their server is within your IP range, your emails could get blacklisted as well. It may seem like a drastic measure by the filters, but spam can cause serious problems.

From Wired Marketing’s point of view, we must be vigilant with our users. If a customer of ours sends spam, it could get our servers blacklisted, causing problems for the rest of our customers.

The sender: Is the sender in my address book, contact list or on a safe sender list?

Some spam filters may block emails if the recipient has never received an email from you in the past; you are seen as a stranger, and you know what they say…never talk to strangers.

If you’re perceived as stranger the spam filter may place your email in the junk folder. When people sign up for your emails, ask them to add your address on their Safe Senders list to ensure the email reaches the inbox every time.

The sender’s ‘from’ address: Is the email address real or fake? Does the address sound too anonymous?

Try to avoid sending campaigns from generic email clients, such as or This appears anonymous to the server. Set up an account with your own domain and build your own reputation.

The sender’s domain name: Is the sender’s domain valid, fake or anonymous looking?

Just because someone is using the domain name, it doesn’t mean it originated from ‘My Great Company’. The only way to link your email domain, with your actual company is by having your email addresses authenticated. This can come in many forms such as SPF, DomainKeys or DMARC – each offers a different level of authentication.

Advice from other spam filters: Has the message or sender been previously reported by other recipients, or other spam filters out there?

The email community also plays a part when the spam filters need to make a decision. The ISP’s users (sometimes hundreds or millions of email users) will receive thousands of emails each day. Whenever someone moves an email into their junk folder, this tells the ISP that the email is unwanted and is classed as an ISP complaint. If the same sender receives multiple complaints, then the ISP can block all future emails from their servers.

In conclusion…

The majority of spam filters are very intelligent and use a combination of the above techniques to determine if a sender is spamming contacts.

Using a spam checker, such as Litmus, email marketers can measure their email before it’s sent. Litmus will give you points and recommendations after analysing your template, and signals if the email is okay to send or if it needs some tweaks.

Don’t worry about using spammy words, phrases, or images – as these are to be expected on some occasions. As long as the rest of your email follows general email marketing best practices, spam filters should let you through.

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