Error Message

Having configured sendmail 8.9.0 and procmail-3.11pre7 on my BSDI
system.

I want mail deliver to /var/mail/$USER
I have quota on for $USER
This works fine, but I want to cutomise the error message, so that
it will notify the sender gracefully.

ie: Error: Receipient mail box is full, please try later etc...

Any suggestion.?