Not to discount George's answer, because generally the PureCloud Conversations API is only available to applications that authenticate via Authorization Code Grant or Implict Grant. However, I did some investigation a few weeks back for similar request and I was able to "read" an email in queue before it was delivered to an agent using Client Credentials Grant. It seems that "read" access is all that you have available however, there is no possibility to perform any other Conversations API operation on the email.
Here are the respective APIs that you should look at:
GET /api/v2/conversations/emails/{conversationId} -> This will return you information about the overall email conversation
GET /api/v2/conversations/emails/{conversationId}/messages -> This will return you a list of all the messageIds in the conversation
GET /api/v2/conversations/emails/{conversationId}/messages/{messageId} -> This will return you the actual text of the email.
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Jim Crespino
Director, Developer Enablement
Genesys
https://developer.genesys.com------------------------------