What are signs that a new filter is active?

Signs that a new filter is active include sudden placement shifts not explained by sending changes, new error messages or header patterns, and corroboration from industry sources reporting similar experiences.

Specific indicators include new SMTP response codes, changed bounce message text, unfamiliar header fields in delivered messages, and altered spam folder placement patterns. These technical artifacts reveal filtering changes even when providers do not announce updates.

Industry corroboration confirms filter changes. If multiple unrelated senders experience similar shifts simultaneously, provider changes are likely rather than coincidental sender issues. Deliverability communities often surface these patterns quickly.