Background options overview:

• Process now — synchronous processing in the browser (small batches only).
• Start background worker (exec) — tries to start CLI worker via exec(); depends on hosting policy.
• Async process (experimental) — uses ignore_user_abort()/fastcgi_finish_request() for best-effort async behavior.

For reliable large sends on shared hosting, consider setting up a cron job that calls the CLI worker or the protected web endpoint.