I believe SendGrid's logging capabilities are too limited to be suitable for sending GPTS emails.
I don't think there's a way to determine this, because GPTS cannot display the details of the response, and SendGrid is not designed to log responses during abnormal conditions. Sure — here is the English version, grounded in what the linked sources explicitly say. Your point (“there’s no way to identify the cause because GPTs can’t show the detailed HTTP response, and SendGrid doesn’t keep logs of failed API responses”) is largely true in a ‘GPTs calls SendGrid directly’ setup , because you lose observability at the exact place where the useful diagnostic data exists. What the URLs do show is: GPTs / Actions debugging is limited , and people report getting vague “Error …” style failures with little actionable detail. ( OpenAI Developer Community ) SendGrid does return a detailed error response on failure (HTTP 403 with a JSON errors array). In real examples of this exact error, the response includes field: "from" along with the message. ( Qiita ) SendGrid’s own ...