list_mailboxes
AI agents call list_mailboxes to retrieve information from Fastmail MCP Server without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
list_mailboxes retrieves and enumerates mailbox metadata without creating, modifying, or deleting data. This is a passive information retrieval operation with minimal blast radius even if misused by an AI agent—at worst, it exposes the user's mailbox structure, which may already be visible to the authenticated client.
From the tool's definition Tool name 'list_mailboxes' combined with server description stating 'read-only access' and listing mailboxes as a supported capability. Description is empty but context confirms it retrieves mailbox information without modification.
Attacks that exploit this kind of access
list_mailboxes. It is categorised as a Read tool in the Fastmail MCP Server MCP Server, which means it retrieves data without modifying state.
Register the Fastmail MCP Server MCP server in PolicyLayer and add a rule for list_mailboxes: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Fastmail MCP Server. Nothing to install.
list_mailboxes is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the list_mailboxes rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for list_mailboxes. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
list_mailboxes is provided by the Fastmail MCP Server MCP server (nova-3951/fastmail-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Every MCP server has a record like this.
Type a name, get the same breakdown: verified identity, auth posture, risk grade, capabilities, recommended policy.
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