Sender Policy Framework or SPF is a new specification that ties DNS and SMTP together to in a sense assume that "all email is SPAM until proven otherwise." I predict that all the mail servers I manage will be using this system within 6 months.
If you run a mail server, I encourage you look into this specification. What occurs is that you publish a list of mail servers that are authorized to send mail for your domain. This publication occurs via the exisiting mechanism of a DNS zone file for your Internet domain name; no new services need to be configured or added for this.
The recieving mail server will merely check to see if the message came from one of those servers. This will allow the message to be discarded if it is forged. (Ever get SPAM from yourself? Well, then you know how easy it is to forge mail envelopes.)
By discarding the messages before doing expensive SPAM checks locally and over the network, the processor cost to scan and deliver mail also decreases.
This should also help lessen the impact of phishers by giving end users some degree of confidence that messages that appear to come from your bank or brokerage actually are!