Override the envelope MAIL FROM with a domain we own when talking to the
upstream relay. SPF passes at the recipient; the attacker's From: header
inside the message body is untouched so they see their own address in their
inbox and believe the relay is real.
First SMTP_PROBE_LIMIT messages per source IP are forwarded via a real
upstream relay (SMTP_UPSTREAM_HOST/PORT/USER/PASS) so the attacker's
test email actually lands in their inbox. All subsequent messages from
the same IP get 250 Ok but only hit the quarantine — campaign content
captured, nothing delivered.
