>My problem is that I don't understand well how filters work. The
>documentation seems to say that the the new message will be in a temporary
>file, and that my REXX script should accept the name of the temporary file
>as the first line parameter. But how will I know the name of the temporary
>file?

At the beginning of your Rexx program, use the command
       PARSE ARG filename recipientlist
Then the variable "filename" will be set to the name of the temporary
file.