https://groups.google.com/g/comp.mail.misc/c/M-pOdrlr6IU
Using Procmail with formail to add headers and forward w/o causing a loop...
https://book.huihoo.com/debian-gnu-linux-desktop-survival-guide/Procmail_Commands.html
Procmail Commands
procmailcommands
https://porkmail.org/era/procmail/mini-faq
Procmail FAQ
A FAQ for Procmail, the Unix mail-processing utility
procmailfaq