reading the envellope

In the following code, how do i read the $1, for the envellope? I think
that the virtualusertables are already set in sendmail. I am actually using
this on a virtual host where anything(_at_)mydomain(_dot_)com goes to the 
mailbox of a
user. And, the code below is in the .procmailrc file of the user's directory.

Thanks in advance

Venky

     DOMAIN=mydomain.com
     ENV_TO=$1

     :0f
     * ENV_TO ?? .
     | formail -i "X-Envelope-To: "$ENV_TO(_at_)$DOMAIN

     :0fE
     | formail -i "X-Envelope-To: UNKNOWN"