mail_upload_attachment · Pyfastmail MCP: Risk & Policy

WHAT IT DOES

What mail_upload_attachment does on Pyfastmail

AI agents use mail_upload_attachment to create or update resources in Pyfastmail — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Pyfastmail environment.

WHY IT NEEDS A POLICY

Why mail_upload_attachment needs a policy

This tool creates or stages new attachment data in the email system, which is a reversible write operation. It does not execute code, delete data irreversibly, or move money.

From the tool's definition Tool name 'mail_upload_attachment' and description 'Upload a blob for use in email composition' indicate creation/staging of email attachment data.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

mail_upload_attachment Rate-limited

mail_upload_attachment stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "mail_upload_attachment": {
      "limits": [
        {
          "counter": "mail_upload_attachment_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

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PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Pyfastmail, so you can run agents against it safely.

FAQ

Questions about mail_upload_attachment

What does the mail_upload_attachment tool do? +

Upload a blob for use in email composition. It is categorised as a Write tool in the Pyfastmail MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on mail_upload_attachment? +

Register the Pyfastmail MCP server in PolicyLayer and add a rule for mail_upload_attachment: 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 Pyfastmail. Nothing to install.

What risk level is mail_upload_attachment? +

mail_upload_attachment is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit mail_upload_attachment? +

Yes. Add a rate_limit block to the mail_upload_attachment 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.

How do I block mail_upload_attachment completely? +

Set action: deny in the PolicyLayer policy for mail_upload_attachment. 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.

What MCP server provides mail_upload_attachment? +

mail_upload_attachment is provided by the Pyfastmail MCP server (pjosols/pyfastmail-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.