Reply to an existing email. Automatically sets correct To/CC, subject (Re: prefix), and threading headers (In-Reply-To, References). Use replyAll=true to include all original recipients. The identityId parameter is optional — if omitted, the server uses the default sending identity.
AI agents use reply_to_email to create or update resources in JMAP MCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your JMAP MCP environment.
This tool sends an email reply, which is a write/create operation that transmits a new message to external recipients. While it creates a new email (Write), it does not delete or irreversibly destroy data. However, the blast radius is high because a misused reply-all could expose sensitive information to unintended recipients or send unwanted communications on behalf of the user.
From the tool's definition Reply to an existing email. Automatically sets correct To/CC, subject (Re: prefix), and threading headers (In-Reply-To, References). Use replyAll=true to include all original recipients.
Documented attack patterns abuse exactly the kind of access reply_to_email gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and JMAP MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for reply_to_email:
{
"version": "1",
"default": "deny",
"tools": {
"reply_to_email": {
"limits": [
{
"counter": "reply_to_email_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}reply_to_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. Automatically sets correct To/CC, subject (Re: prefix), and threading headers (In-Reply-To, References). Use replyAll=true to include all original recipients. The identityId parameter is optional — if omitted, the server uses the default sending identity. It is categorised as a Write tool in the JMAP MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the JMAP MCP server in PolicyLayer and add a rule for reply_to_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 JMAP MCP. Nothing to install.
reply_to_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_to_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_to_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_to_email is provided by the JMAP MCP server (wyattjoh/jmap-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 11 JMAP MCP tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
11 JMAP MCP tools catalogued and risk-classified — across an index of 42,500+ MCP servers.