Definition
Filtering incoming email traffic based on specific criteria.
How it works
Mail filters can be implemented to scan inbound email messages at the initial SMTP connection stage to detect and reject email containing spam and malware.
This technique is distinct from d3f:EmailDeletion because it prevents an email from reaching an user's inbox. This technique can also be used for outbound email traffic.
Considerations
- The effectiveness of mail filters depend on the completeness of the filter policies
Implementation perspective
Email Filtering should be treated as a technical defensive capability rather than a product checkbox. In practice, teams should define the protected scope, the conditions under which the technique acts, and the observable evidence that demonstrates the intended behavior. For this technique, likely engineering context includes Email.
Use the technique to create a logical or physical boundary that limits access, execution, communication, or movement.
Questions to ask
- What is being isolated, from whom or what, and under which conditions?
- Which flows or operations remain explicitly allowed across the boundary?
- What happens when the isolation mechanism fails or loses policy context?
- How is the boundary tested for alternate paths, bypasses, and unintended dependencies?
Evidence and validation
- Architecture and data-flow diagrams showing the isolation boundary
- Policy, rule, or configuration exports from enforcing components
- Tests of allowed, denied, and failure-state behavior
- Monitoring evidence for boundary violations and policy changes
Common failure patterns
- The intended boundary has undocumented alternate paths or inherited trust relationships.
- Rules accumulate without ownership, review, or removal of obsolete exceptions.
- Isolation is assumed from network location while application, identity, or management paths remain open.
This implementation perspective is original Bare Metal Cyber educational content. It does not replace the official D3FEND definition or establish that a specific product implements the technique.
Technique hierarchy
Top-level family
Parent techniques
Direct child techniques
None listed at this level.
Artifacts and ontology entities
These relationships describe how D3FEND connects a defensive technique to artifacts or other ontology entities. They describe graph semantics, not a product certification.
Explicit technique relationships
Show inferred artifact relationship paths (1)
Offensive-technique relationships
These relationships are generated from D3FEND graph paths and are explicitly experimental. They should be treated as hypotheses for defensive analysis—not as proof that the technique prevents, detects, or removes an offensive behavior.
ATT&CK Enterprise (4)
ATT&CK for ICS (1)
Authoritative sources
- Open this technique on the official D3FEND website ↗
- Open the official ontology resources ↗
- System and method for providing anonymous remailing and filtering of electronic mail ↗Nokia of America Corp · Eran Gabber, Phillip B Gibbons, David Morris Kristol, Yossi Matias, Alain J Mayer · PatentReference
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