[RFC] Architecture questions for an event-driven Email MCP Server

[radiykharisov]

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Discussion Topic

Building an MCP translation layer for Atomic Mail (our event-driven email backend running on Haraka + Kafka + Cyrus). The goal is to expose it as a native email infrastructure for agents, but we are hitting a few API design questions regarding MCP best practices:

1. The backend is event-driven via Kafka. What's the current consensus on notifying agents about new mail? Polling an MCP tool feels terribly inefficient. Is there a solid server-to-client push pattern evolving in MCP?

2. For attachments, we drop heavy files into S3 to avoid huge payloads (claim-check pattern). When an agent needs to read a file, is it better practice to just return a pre-signed S3 URL via a tool (for out-of-band fetch), or pipe the data directly through the MCP response?

3. We have native JMAP support. In your experience, do LLMs actually handle deeply nested JMAP JSON reliably? Or is it better to abstract it and flatten everything into narrow, task-specific tools (e.g., get_clean_thread) to save context?

Would love to hear how others are handling similar data-heavy or event-driven setups.