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Do not build this if several people share an inbox all day. Collision detection is the product.

Two agents answering the same customer at once is the failure a shared mailbox produces on its first busy morning, and preventing it needs live presence and a warning at the moment of sending. Add email threading that survives every client's quoting style — far harder than it looks — and this is a real project rather than a weekend.

WHEN TO KEEP PAYING
- More than two or three people working the same address every day
- Anywhere a duplicate reply is embarrassing rather than merely untidy

WHEN TO BUILD
- A small team, or a support inbox where assignment is enough. The full shape is under Help Scout elsewhere in this catalogue

THE TWO THINGS THAT DECIDE WHETHER IT WORKS

**Threading.** Match on Message-ID, then In-Reply-To, then References, and only then fall back to subject plus participants. Never subject alone — two customers writing 'Re: my order' must not become one conversation. Strip quoted history for display and keep the whole message, with a fixture per mail client because every client quotes differently. Detect automatic replies and bounces from their headers and never turn them into conversations, or the inbox starts talking to itself

**Collision.** Record who has a conversation open and who is typing, shown to everybody else. A warning before sending if another reply went out while yours was being written. It needs no socket layer — a small polling endpoint is enough and far less to operate

AND THE ONE THAT IS UNRECOVERABLE
An internal note must be impossible to send to the customer by accident. Render the two so differently they cannot be confused, and default to internal. That mistake is seen by the person it is about, and no apology fixes it.

WATCH FOR SILENCE
An inbox that quietly stops receiving is the worst failure here. Monitor the age of the newest inbound message and alert on it.

THE ONE-LINE VERSION
Buy it for a busy shared address. If you build it, spend the time on threading and on making a private note impossible to send.