Pretty much all the spams that I receive contain a link to a Web site where
you are supposed to go for more information.  I noticed that very often,
these Web sites have been registered recently (say, in the past two months to be
conservative, but it’s more like a couple of weeks).  The reason for this
fast turnover is that spam domain names get taken down typically in the weeks
that follow the spam itself.

I was wondering if this couldn’t be used by filters (either Bayesian or
simple rule filters such as SpamAssassin) to increase the spam probability of a
suspicious email.

Whenever a domain name included in an email has been registered in the past
two months, mark it as spam.

Could it be so simple?