You may receive error
500 ACTION_FAILED API when calling API endpoint "/api/v1/cases/posts/:id/email_original.json".
This is normal behavior when you try to query the email information for replies that were added via the Agent Area/Support portal instead of using email channel. In other words, there was not email received for this conversation reply as it was added from Kayako Web UI.
In other words, this API is designed to only provide the original email for posts that were sent via an email client, not the Agent Area. Therefore, if you are trying to access the original email of a post that didn't have an original email, you will encounter this error. To avoid this, ensure that you are only trying to access the original email for posts that were sent via the email channel.
You can doublecheck the behavior from Kayako agent are, you can see that there is no prompt in in the top right corner of the reply
Show original email when it was added from the support portal.