One AI, Every Mailbox - Our Own Outlook Connector

We run more than one business, and each has its own Microsoft 365 organisation and its own mailboxes. The working day was spent hopping between them: check one inbox, then another, copy the important bits into Trello, and write replies that sound like us.

We wanted to ask one AI "what needs my attention this morning?" and have it answer across every mailbox, then act on the answer.

Claude's own Microsoft 365 connector is good at searching. But it connects one Microsoft account at a time, its write tools have to be switched on by an admin, it can't attach files, and it has no tool to move an email to a folder other than Deleted Items. ChatGPT's Outlook app sends plain-text email after you approve each draft, and each extra account is a separate connection.

None of them could see two separate Microsoft 365 organisations at once, and none could hold our own rules about how an email should be written.

A custom connector, using the Model Context Protocol (MCP) that Claude and ChatGPT both support. It signs in to each organisation with its own Microsoft permissions, so every mailbox is reachable from one conversation. Each mailbox keeps its own signature, and the AI always has to say which mailbox it is sending from.

It runs on our own computer, beside Claude Desktop and Claude Code, so there is no server to host and no monthly cloud cost.

Telling an AI how to write only goes so far. So the rules sit in the connector itself. Before a send, reply or forward goes out, the text is checked: length limits, banned stock phrases and house style. If it fails, the email is not sent and the AI is told exactly what to fix. Only an explicit request for a longer email lets it through.

The same idea works for any business: approval rules, words you never use, who can be emailed from which mailbox, or which emails always need a person to look first.

• Microsoft Graph: app-only access per tenant, with a sign-in fallback where an organisation needs it.
• Model Context Protocol: one server, 24 tools, used from Claude Desktop and Claude Code.
• Security: secrets in the operating system keychain, nothing stored in plain-text config.
• Reliability: automatic retries when Microsoft throttles, and a test suite covering mailbox selection, replies, forwards, attachments and the writing rules.

The day starts with one briefing across every mailbox: unread counts, flagged mail and the next 24 hours of meetings. Anything that needs action becomes a Trello card in a couple of clicks, and replies go out from the right mailbox, with the right signature, in our own voice.