Why do forwarded emails create a control gap in hybrid mail environments?

Why This Matters for Security Teams

Forwarded email is not just a delivery convenience. In hybrid mail environments, it can become a second path for sensitive content that bypasses the controls tied to the original mailbox. That matters because message inspection, retention, revocation, and remediation are often designed around a cloud mailbox boundary, while downstream forwarding, journaling, shared mailboxes, and transport rules can create copies that persist elsewhere. The result is a control gap between where the message first arrived and where it is actually consumed.

This is especially relevant when security teams assume that deleting, quarantining, or retroactively remediating the source message removes all exposure. In practice, the forwarded copy may already exist in another mailbox, archive, ticketing system, or workflow tool. The difference between a controlled inbox and an uncontrolled copy is often invisible until an incident review. NHI Management Group’s Ultimate Guide to NHIs — Standards is useful here because it frames how identity-bound controls need to follow the message path, not just the primary mailbox. Forwarding also weakens the assumption that mailbox-level policy is enough, a pattern that aligns with the broader control failures seen in the DeepSeek breach research, where exposed content remained accessible outside the intended control plane.

In practice, many security teams discover forwarded-message exposure only after data has already propagated into systems they do not monitor as closely as the primary mailbox.

How It Works in Practice

The gap usually appears when a message is processed by more than one mail path. A cloud mailbox may be covered by quarantine, DLP, retention, or eDiscovery, but a forwarded copy can land in an on-premises mailbox, a shared mailbox, or a downstream service that is not wired into the same remediation workflow. Hybrid Exchange often increases this risk because routing rules, connector behavior, and mailbox delegation can split control across administrative domains.

Practically, the defensive model needs to account for the message lifecycle, not only the inbox. That means tracing where forwarding is allowed, where copies are created, and whether the security team can still act on them after delivery. The NIST Cybersecurity Framework 2.0 is relevant because it emphasizes governance, asset visibility, and response coordination across systems. For email-specific handling, teams should also map controls to:

• server-side forwarding rules versus user-created inbox rules
• shared mailbox and delegated access paths
• journaling, archiving, and compliance capture points
• mail flow connectors between cloud and on-premises Exchange
• retention and purge capabilities in every location that stores a copy

The most reliable operational approach is to treat forwarding as a data replication event and enforce policy where the copy is created, not only where the original message was received. That usually requires central review of forwarding rules, alerting on new external routes, and confirming whether security tooling can redact, revoke, or quarantine forwarded copies across both environments. When messages move through legacy relay paths or third-party archiving systems without policy hooks, these controls tend to break down because the remediation authority no longer follows the message.

Common Variations and Edge Cases

Tighter forwarding controls often increase operational burden, requiring organisations to balance user flexibility against message containment. That tradeoff becomes sharper in shared mailboxes, legal hold scenarios, and executive assistants’ workflows, where forwarding may be legitimate but still expands the exposure surface.

Current guidance suggests treating external forwarding differently from internal delegation, but there is no universal standard for this yet. Some environments block all external auto-forwarding by default, while others allow exceptions with review. The decision depends on whether the business process can tolerate message copies leaving the primary control plane. A forwarded message can also inherit enough context to create a secondary incident, even when the original message is later removed or quarantined.

Hybrid deployments need extra scrutiny because mailbox policy, transport policy, and archive policy may not be managed by the same team. That is where visibility gaps form. If the organisation uses shared mailboxes for operations, finance, or customer support, the team should verify that forwarded copies are included in DLP and retention scans, not assumed to be covered by the source mailbox policy. The broader lesson from NHIMG research is that identity and content controls must be validated at every handoff, not only at the initial point of receipt. The research on standards for NHIs reinforces that control integrity depends on the path, while the DeepSeek breach underscores how quickly sensitive material persists once it escapes the intended boundary.

Related resources from NHI Mgmt Group

• Why do non-human identities create audit risk in modern environments?
• Why do hybrid identities create blind spots in existing IAM programmes?
• How should security teams prioritise NHI remediation in cloud environments?
• Why do non-human identities create more audit risk than human accounts?