Reply to an existing email with proper threading headers (In-Reply-To, References). Automatically fetches the original email to build the reply chain. By default sends immediately; set send=false to save as a draft instead.
AI agents use reply_email to create or update resources in Fastmail MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Fastmail MCP Server environment.
This tool sends email replies over the internet to external recipients. While sending an email is not strictly destructive or financial, it has high potential for misuse by an AI agent (e.g., sending unauthorized or harmful replies impersonating the user). It is a Write operation that externalizes data and has real-world communication consequences, making the severity high.
From the tool's definition Reply to an existing email with proper threading headers... By default sends immediately; set send=false to save as a draft instead.
Documented attack patterns abuse exactly the kind of access reply_email gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Fastmail MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for reply_email:
{
"version": "1",
"default": "deny",
"tools": {
"reply_email": {
"limits": [
{
"counter": "reply_email_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}reply_email 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.
Free to start. No card required.
Reply to an existing email with proper threading headers (In-Reply-To, References). Automatically fetches the original email to build the reply chain. By default sends immediately; set send=false to save as a draft instead. It is categorised as a Write tool in the Fastmail MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Fastmail MCP Server MCP server in PolicyLayer and add a rule for reply_email: 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.
reply_email is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the reply_email 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 reply_email. 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.
reply_email is provided by the Fastmail MCP Server MCP server (madllama25/fastmail-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Fastmail MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
38 Fastmail MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.