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 documentation (JP) states that when a request fails, the API returns a response code plus a detailed error message and related-document links — i.e., the diagnostics exist in the API response. (SendGrid)

So: the diagnosis signal exists, but if GPTs doesn’t surface it and you don’t capture it elsewhere, you effectively can’t see it.

Practical ways to still pinpoint the cause (based on the sources)

1. Check what SendGrid considers “verified/allowed senders” via API and compare to your From

• SendGrid Support explains the cause: this happens when you try to send from an address that is not a Verified Sender (or not using an authenticated domain) in the From field. (support.sendgrid.com)

• There is an endpoint to retrieve Sender Identities and it explicitly says it returns both verified and unverified senders. (Twilio)

2. Insert a thin “proxy” API between GPTs and SendGrid to log the raw error response

• The error details (including field: "from") are shown in real-world 403 responses. (Qiita)

• Because GPTs action debugging is limited, logging the outbound request and inbound SendGrid response at a proxy is the reliable way to retain the diagnostic payload. (OpenAI Developer Community)