Using server-to-server mail delivery via Gmail REST API in Java, I face a ‘400 Bad Request: Failed Precondition’ error. What might be causing this problem?
I encountered a similar issue while setting up server-to-server communication with the Gmail API. In my case, the error resulted from an improperly configured service account, particularly regarding domain-wide delegation. I double-checked that the service account had been granted the necessary permissions on the Google Admin Console, ensuring that the scopes matched what was required for mail delivery. It is also important to verify the sender’s email authorization, as mismatched credentials can trigger the failed precondition error. Reviewing these aspects helped me resolve the problem.