Your keywords are maildropfilter and xfilter.
You have a cascade of postfixes and aliases your mails are traversing through and are losing the original recipient address? Simply add a header line with first servers postfix. Tune your main.cf and add something like the ‘detect_recipients‘ line here: smtpd_recipient_restrictions = check_recipient_access regexp:/etc/postfix/specials/denied_recipients, check_recipient_access regexp:/etc/postfix/specials/allowed_recipients, check_recipient_access regexp:/etc/postfix/specials/detect_recipients, check_recipient_access regexp:/etc/postfix/specials permit_mynetworks, reject_unauth_destination Your detect_recipients file: […]
Add your custom header. Do a folder repair. Thats all.