A spam filter is software that checks incoming email and keeps unwanted or dangerous messages out of your inbox.
A spam filter checks every incoming email and decides whether it belongs in your inbox, your junk folder or nowhere at all. It looks for the patterns that unwanted mail tends to share: suspicious senders, misleading links, known scam wording and messages that fail authentication checks.
Most spam filters don't make a simple yes or no decision. They give each message a score based on many small signals, and anything above a threshold you can set is marked as spam or rejected. A newsletter you signed up for might score low, while a fake invoice from a domain registered yesterday scores high.
How a spam filter works
Spam filters combine several checks:
- Sender reputation: is the sending server on a public blocklist?
- Authentication: does the message pass SPF, DKIM and DMARC checks for the domain it claims to come from?
- Content rules: does it contain phrases, links or attachments common in spam and phishing?
- Learning: has the filter been trained on messages people marked as spam or not spam?
Each check adds or subtracts points, and the final score decides what happens to the email.
Spam filter settings you can change
On most hosting accounts you can adjust the filter from your control panel or webmail:
- Raise or lower the spam score threshold
- Send spam to a separate folder instead of deleting it
- Allowlist senders you always want to receive
- Blocklist addresses or domains that keep getting through
Start with spam going to a folder, check it for a week or two, and only then make the filter stricter.
Common spam filter problems
The two failures are false negatives, where spam reaches your inbox, and false positives, where real email ends up in junk. False positives often hurt more, because you might miss an order or an invoice. If genuine mail keeps getting caught, allowlist the sender and mark the message as not spam so the filter learns.
Spam filters and email deliverability
Spam filters also decide whether your own emails reach other people. If your domain lacks SPF and DKIM records, or your server sends unusual volumes, your messages may be flagged by the recipient's filter. That's the subject of email deliverability. Setting up the right DNS records for your domain, starting with correct MX records, is the first step.
Spam filters vs firewalls and malware scanning
A spam filter deals with email. A firewall or WAF deals with traffic to your website, and malware scanning looks for harmful code in your files. You need all three, because each one guards a different door.