For your use case I would add your own Message-ID MIME header to the outgoing message. When a user replies, usually (in most cases) you'll see the ID of the ori

For your use case I would add your own Message-ID MIME header to the outgoing message. When a user replies, usually (in most cases) you'll see the ID of the original message as In-Reply-To header. And it will be posted as In-Reply-To HTTP parameter to your app. There's also "References" MIME header you can use. I hope this helps.