bsd-mailx(1) on Linux
The text fa4f9374166a, as Debian 11 (Bullseye), Debian 10 (Buster) shipped it.
send and receive mail
Flags it lists
-
-- -
-E -
-I -
-N -
-a -
-b -
-c -
-d -
-f -
-i -
-n -
-r -
-s -
-u -
-v