Architecture

src/core/             Address CRUD, rate limiting, dispatch, MIME parsing.
                       Doesn't import from adapters/ or storage/.
src/core/storage.ts    SqlExecutor: the run/first/all interface core runs
                       SQL against. D1 and SQLite are both SQLite dialect,
                       so core/db.ts and core/ratelimit.ts are shared
                       between them word for word.
src/storage/           d1.ts and sqlite.ts, the two SqlExecutor drivers.
src/adapters/          Delivery adapters. discord/ ships built in.
src/worker.ts          Cloudflare entrypoint.
src/node/               Self-hosted entrypoint (SMTP server, HTTP server
                         for the Discord webhook, cleanup schedule).
src/receivers/mailtm/  mail.tm entrypoint (API client, poller instead of
                         SMTP, its own cleanup that deletes the mail.tm
                         account before dropping the row).

Extending it

Delivery adapter. Implement MailAdapter in src/core/types.ts: a name, and a deliver(owner, mail) that returns { success, error? } and never throws. Register it in buildAdapters().

Storage or receiving backend. Implement SqlExecutor in src/core/storage.ts. Call handleInboundEmail({ to, from, raw }, db, dispatcher) from core/email.ts for each piece of mail. raw takes a Buffer, ReadableStream, or string, whatever postal-mime accepts.