How it works: the worker auto-detects the email column,
resolves each domain's MX host, and paces probes per MX (~4s for small
hosts, ~6s for Google/Microsoft) with adaptive backoff on deferrals.
Rows marked invalid are dropped; everything else (safe / risky
/ unknown) is kept. Daily cap is 10,000 verifications — larger files
spill across days, one downloadable CSV per UTC day. Output rows are in
completion order, not input order.