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Spam Filters

A spam filter is a program, device, or algorithm that identifies unwanted, unsolicited, or potentially dangerous email messages (commonly known as “spam”) and prevents them from getting dropped into your email inbox. Internet Services Providers, or ISPs, use spam filters to ensure that they are not contributing to spam distribution. Spam filters can vary between mailbox providers, companies, and even individuals.

How Do Spam Filters Work?

Spam filters use a variety of criteria to decide if an incoming email should be flagged as spam. After scanning each message, the filter assigns a spam score, which determines if the email should pass through or remain in the filter. Scores vary between servers.

There are several different categories of mail filters.

  • Content filters look for suspicious words inside an email or subject line.

  • Bayesian filters rely on the user’s specified spam preferences to learn and adapt over time.

  • Rule-based filters allow for certain types of messages to filter through based on user-prescribed rules.

  • Permission filters require the sender to be pre-approved by the recipient.

  • Challenge-response filters ask the sender to enter a code that grants permission to send the message.

  • Blacklist filters stop any message from coming through that was sent from a known spam sender.

Mail filtering solutions can be deployed in various ways, from a hosted or “cloud” solution to an on-premise device or an installed software program that integrates with the user’s email client.

Why Do Spam Filters Exist?

Spam is an annoyance at best and a security risk at worst. Although it may seem like it should be up to the user to decide which messages are spam and which are worthy of opening, spam filters are designed to protect the user and provide a better experience. Mailbox providers have competitors just like businesses in any other industry. They want users to trust and enjoy their experience, so they don’t switch to another provider. A large part of that experience is ensuring that malicious messages never made their way through to a user’s account, where they could potentially compromise security or spread a virus.

How Do Spam Filters Effect Email Deliverability?

When an email gets caught in a spam filter, deliverability rates naturally go down. Even when the message is desired or requested by the sender, certain factors may keep it from reaching its intended recipient. Email senders can help guard against the spam filter trap by:

  • Securing permission or an “opt-in” from the recipient

  • Sending messages from a dedicated IP address

  • Including an unsubscribe link to maintain compliance with spam laws

  • Being mindful that email and subject line text avoids spam trigger words

  • Practicing inbox warming before launching an outreach campaign

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