The busybox sendmail help contains: "Other options are silently ignored; -oi is implied." But sendmail does not ignore other options: sendmail -F aaa sendmail: invalid option -- 'F' BusyBox v1.32.0 () multi-call binary. Usage: sendmail [-tv] [-f SENDER] [-amLOGIN 4<user_pass.txt | -auUSER -apPASS] [-w SECS] [-H 'PROG ARGS' | -S HOST] [RECIPIENT_EMAIL]...