Allow local spam weight override on domain level

[isgsoftware]

Our use case is that a particular customer is having messages from a mail chimp list blocked due to baruwa URL blacklist with high weight of 10.00. Whitelisting is not effective because the subdomain & hostnames of the mail servers change all the time. We don't want to lower the global weight because then other customers will likely get more spam, creating more support requests.

The ideal feature would allow us to lower the weight of a spam test for a particular domain. This way we can tune the spam filter for one customer without affecting all customers.