A spam filter evaluates incoming email for unwanted or suspicious content. Email providers, company gateways, and email clients can filter at different points, using their own rules and signals.
What signals can filters consider?
Filters may consider the sending IP address or domain, authentication, message content, links, and known signs of abuse. A blocklist or allowlist can affect the assessment. Which signals count and how much weight they receive depends on the service.
What happens to a flagged message?
A receiving server can reject a message during transfer. If the service accepts it, it can label it, quarantine it, or place it in the spam folder. Legitimate messages can also be misclassified.
Example of an authenticated email
A newsletter passes authentication but links to a domain the provider considers suspicious. The provider can still place the newsletter in spam. In a case like this, first check whether the server accepted or rejected the message, then investigate the link and other signals.
