Sparse still indexes a row that has `to` but no messageId — every inside letter — so the compound unique index collided on the first two and took a real letter down with it. Partial, on rows that carry a messageId, instead. Google's SMTP relay closes at EHLO unless the greeting names a real host and TLS comes first; give it both, and fall back to plain Gmail SMTP (which keeps the name and Reply-To, rewriting only the From address) if it still refuses. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>