Batch off = one email per request, log fills live. Batch on = one request per N emails, reuses one SMTP connection — only worth it with External SMTP. Above ~6 threads you need HTTP/2 on your server, or list multiple endpoint URLs. Queue only should stay ON unless you need per-address rejections in the log.
The tab must stay open. The countdown keeps correct time across tab switches and sleep — it starts the moment it reaches zero.
Credentials are tested before saving.