Gmail OAuth client ID and secret are required
The mailer tried to build a Gmail transport and found missing OAuth client ID or secret. Shorter sibling of Gmail OAuth client ID and secret are required (Admin → Settings → Email) (config assert / health).
Why this happens
mode === "gmail" but xoauth2ClientId / xoauth2ClientSecret are empty at send time. Distinct from missing refresh token (Gmail is not authorized siblings).
Diagnose and fix
1. Admin → Settings → Email — Gmail mode; paste Google OAuth client ID and secret; save.
2. Set the Google account email, then Connect Google for the refresh token.
3. Follow Gmail XOAUTH2 for Cloud Console setup.
4. Or switch to SMTP until Gmail OAuth is ready.