hashcash-payment-alist

hashcash-payment-alist is a variable defined in `hashcash.el'.
Its value is

nil

Documentation:
An association list mapping email addresses to payment amounts.
Elements may consist of (ADDR AMOUNT) or (ADDR STRING AMOUNT), where
ADDR is the email address of the intended recipient and AMOUNT is
the value of hashcash payment to be made to that user. STRING, if
present, is the string to be hashed; if not present ADDR will be used.

You can customize this variable.