The Send Email action also allows to reply to emails, so that the email that is being sent is added to an email thread.
To get started with that functionality, we highly recommend to use this template flow, as it requires some advanced knowledge to configure the details on your own, due to the nature of how email works.
In order to reply to emails the following fields need to be filled as specified:
Subject: The subject must match the subject of the email that is being replied to
Thread ID: The ID of the thread the message belongs to
In-Reply-To and References Address: This must be set according to the Message-Id of the email that is being replied to
Additional field for replying to emails
In order to retrieve these required values, the Get Message action should be used, as it returns all these values. A working example, which also handles some non-standard email client implementations can be found here.
Not all email clients follow the same standards regarding headers, thus there might be slight differences in the header names for each email, this is e.g. the case for the Subject or Message-Id header.