Untracked replies in the master inbox will show up with a button to Attach Contact. This action can be done on a larger scale using the API.

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Getting all scheduled emails for a lead

Send a GET request to the following endpoint 
/api/scheduled-emails/{lead_id_or_email}

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Attaching scheduled emails to a reply

Once you have the scheduled email IDs you can attach them to replies with their ID. Send a POST request to the following endpoint 
api/replies/{reply_id}/attach-email-to-reply
The following are the parameters for the request.
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scheduled_email_id
integer
required
The ID of the scheduled email you to attach to the reply.