Salesforce API calls

Hello Community,

Customer is using the Rest API package from the Bot to perform certain salesforce API calls related to email communication of salesforce. These API's are supposed to append to existing email records in salesforce, 

The API's itself work fine and bot is not experiencing any issue but the problem is that every time it created a new email record instead of appending to existing email chain. Any suggestions on the best APIs that can be used in this situation?