thanks. i thought about hidden field.

I don't want user manually creat the form. so i will write another script (config.pl) that let user input domain/email and save it into a config file. it will be read into the formmail form and will be the content of the hidden field.
in the config.pl,i have to depend on the $ENV{DOCUMENT_ROOT} to parse the user name, then the config file will store in users dir.

is it safe to use $ENV{DOCUMENT_ROOT} ?

btw, i am using nms' improved Matt scripts. so it's more secure.