[wlug] Spam and encrypted emails

http://www.clear.net.nz/terms.html Clear have advised that spam from known spam addresses will be blocked and that other messages will be quaranteened for only 3 days after which the messages will be deleted. As I am not a systems administrator I was wondering what the usual policy for encrypted email (eg GPG) in regard to spam filtering. Is encrypting an email a guarenteed way for people to communicate and avoid messages being deleted by aggressive ISP spam policies. I assume spammers wouldn't bother encrypting email because receiver's couldn't decrypt the emails. Thoughts? Chris