Hello,

We are integrating Mailpit into our testing environments where messages are captured before being relayed to end users.

Our backend system (Mailman) generates a unique Message-ID and stores it in OpenSearch for tracking. When users reply to those emails, Mailman uses the In-Reply-To header to associate replies with the original sent message via its Message-ID.

Currently, when releasing a message via Mailpit’s UI/API, a new Message-ID is assigned. This breaks our threading because the In-Reply-To header no longer points to any known Message-ID in our system.

We understand this behavior is by design for testing Gmail, etc., but we’d like to request a configuration option to preserve the original Message-ID when relaying/releasing.

Why this matters:

1. We want to avoid reverting to Mailhog, which is no longer maintained.

2. Accurate threading is critical to our QA and automated testing processes.

3. Mailpit is actively maintained and this would make it a robust fit for systems that depend on Message-ID continuity.

Kindly advise, and thanks for your awesome work on Mailpit!