Mailbox Permissions

How mailbox permissions work

Mailbox permissions are authorization rules on a messaging resource. They determine whether a person, service account, or application can read mailbox content, send mail as the mailbox, manage settings, or delegate access. In practice, they are one of the controls that translate directory membership and application trust into concrete mailbox access.

Because email remains a high-value business channel, mailbox permissions often sit at the boundary between collaboration and control. They can be granted directly, inherited through groups, or exposed through delegated access and application scopes. That flexibility is useful, but it also means the access path is easy to misunderstand if the permissions model is not documented and reviewed.

Why mailbox permissions matter for security

Mailbox permissions can expose message content, contact data, calendar details, and sent-mail capability. When they are broader than intended, a mailbox can become a convenient pivot point for data exposure, impersonation, or covert monitoring. This is especially important where administrative convenience has overridden least privilege.

Mailbox access is also interesting to attackers because email commonly contains password resets, internal approvals, and business context. If an account, token, or delegated grant is abused, the mailbox can reveal both sensitive information and paths into other systems. The OWASP Non-Human Identity Top 10 is relevant here because overprivilege and secret handling problems often show up in the same environments that expose mailbox access.

Mailbox permissions also matter when non-human actors are involved. Service accounts and agents may need mail access for routing, automation, or alerting, but that access should be narrow and traceable. AI Agent Authorisation Guide is a useful companion when the mailbox is being accessed by an autonomous or semi-autonomous agent.

Common permission patterns and failure modes

Mailbox permissions usually appear in a few recurring patterns: direct user access, shared mailbox access, delegated send-as or send-on-behalf rights, and application access through mail scopes. Each pattern solves a legitimate operational problem, but each also creates a different trust boundary and a different abuse path if it is too broad.

The most common failure modes are excessive access, stale access, and unclear ownership. A permission may remain in place after a role change, or a shared mailbox may be left accessible to a broad group long after the original business need has ended. Over time, these grants create invisible data paths that are hard to detect in a large directory.

Mailbox permissions are therefore closely related to Privileged Access Management Guide and Authorisation Models Guide, because the central question is not only who can access the mailbox, but how that access is expressed, reviewed, and constrained.

How to review and govern mailbox permissions

A useful review starts with a complete inventory of mailboxes, delegated grants, service principals, and group-based access paths. From there, the key question is whether each permission is still needed, whether it is granted to the right subject, and whether the level of access matches the business function.

Governance is stronger when mailbox permissions are reviewed as part of joiner, mover, leaver, and application lifecycle processes rather than as a one-off cleanup. That helps catch lingering delegated rights, dormant shared mailbox access, and application scopes that no longer match the current operating model. For cloud-based messaging, Cloud PAM and CIEM Guide is directly relevant because effective permissions and right-sizing are the same governance problem expressed in a cloud identity context.

Where mailbox access is used for privileged operations, especially in large cloud or hybrid estates, the right model is usually least privilege with time-bounded elevation. Just-in-Time Access and Zero Standing Privilege Guide helps frame that approach for access that should exist only when needed, rather than continuously.

Risk and Threat Considerations

Mailbox permissions can create both exposure and persistence. Overbroad delegated access may let an attacker read sensitive correspondence, impersonate a sender, or quietly monitor a mailbox for password reset links and approval workflows. Stale grants are especially dangerous because they often survive role changes and are easy to overlook during routine administration.

Failure mechanism: A mailbox permission grants more access than the current business need requires, or a delegated right remains active after the legitimate user, app, or role has changed, giving an attacker or insider a durable access path.

Impact: The mailbox can become a channel for data theft, impersonation, business-email abuse, and downstream compromise of other systems that rely on email for authentication, notification, or approval.

Practitioner Guidance

Governance implication: Treat mailbox permissions as a reviewed authorization surface, not as a convenience setting. Ownership should be explicit, and every non-default grant should have a current business purpose, a named approver, and a clear expiry or review cycle.

What to watch for: Pay particular attention to broad group grants, delegated send rights, application scopes, and service accounts with mailbox access. Those are the permission paths most likely to drift from intended use and create hidden exposure.

Practitioner takeaway: If a mailbox can read, send, or delegate on behalf of others, the permission should be justified with the same rigor you would apply to any other privileged access.

Related resources from NHI Mgmt Group

• How do mailbox permissions turn a valid login into broader compromise?
• Why do compromised non-human identities and elevated mailbox permissions make email attacks harder to contain?
• Who is accountable when mailbox permissions, tokens, and browser controls are not reviewed after an intrusion?
• What are the common mistakes teams make when using application permissions for mailbox investigation and cleanup?