How to Stop Your Emails from Being Blocked as Spam

  Jeroen Seynhaeve     2025-08-13 How to Stop Your Emails from Being Blocked as Spam

If you’ve ever sent an important email and it mysteriously never reached the recipient’s inbox, there’s a good chance it got caught in a spam filter. Let’s look at what spam is, how spam filters work, and—most importantly: the best practices you can follow to keep your legitimate emails from being marked as spam.

What Is Spam?

Spam is defined as email sent to a large number of recipients without their explicit permission, typically with the goal of prompting a commercial action, such as buying a product or service. Examples include mass marketing emails sent to purchased mailing lists or promotional messages to people who never opted in.

How Spam Filters Work

Mail servers use spam filters to detect and block unwanted messages before they reach inboxes. These filters rely on a mix of rules, keyword checks, and behaviour analysis to identify suspicious emails. Common spam red flags include certain words and phrases (e.g., “Buy Now” or “Get Paid”), the overuse of punctuation, capital letters, or large fonts, and certain hyperlinks (urls), images, and unusual formatting.

Spam filtering techniques are constantly evolving to keep up with new spam tactics, and spam filter administrators keep their exact techniques secret to prevent spammers from bypassing them. This means even perfectly crafted, legitimate emails can still end up in spam folders.

Best Practices to Keep Your Emails Out of Spam