Tech Tips

July 27, 2014

How Spam Filters Decide to Block Your Emails

What identifies an email as spam? Mail servers will scan for certain words, phrases or email elements to catch spam. Sometimes they grab a legitimate message because it has a few of those characteristics. Here's how to avoid getting caught.
What makes a spam filter pick out certain legitimate emails and say, “This is spam!”? It happens to everything from that critical report, to this very newsletter. Seemingly at random.

Or is it?

Spam filtration is governed by a number of set criteria, or “triggers”. The filters investigate every email for these triggers; if an email meets enough of them, it’s blocked. Most of the time it works as intended. Sometimes though, it just messes up.

If you want to avoid having emails end up in Junk Mail or Quarantine, it pays to know what those triggers are. And NOT to use them!

Depending on the mail server, triggers can include:
  1. Certain words/phrases in the email (see the big list below)
  2. Certain words in the subject line (like "Viagra" or "Refinance")
  3. Ratio of text to images – lots of images + very little text = spammy!
  4. An empty message – for example, one big image and no text at all.
  5. A background image tiled across the entire email (Outlook does let you do this, but don’t!)
  6. A large number of symbols in the email text, such as exclamation points (!) or ampersands (&).
  7. Reputation of URLs in the message – If a URL is “known” as bad on the Web (e.g. associated with malware or phishing), a spam filter will block the email
  8. Certain email attachments, like program (.exe) files
  9. The domain from which the email is sent – If a domain is often used to send out spam, viruses or malware, spam filters will block emails from that domain
  10. No unsubscribe link
(Thanks to Vircom for providing additional triggers in response to our request.)

Here’s a long list (but still incomplete!) of email spam trigger words from Hubspot: The Ultimate List of Email SPAM Trigger Words

Don’t use any of these and your emails should reach their destination unscathed. Want to check first? Use an email spam test tool, like this one: