There is, in fact, very little you can do to stop this in the general case.
In specific, signing e-mail that has official communications is a good idea. Several computer and software vendors (including both Microsoft and Apple) do this. However, if there is someone on the Net who specifically wants to send e-mail spoofed from your company, there's nothing you can do to stop it. E-mail is a worldwide, decentralized system. An open server in Elbonia can send e-mail purporting to be from you or me. This is why spam-fighters want to see all the e-mail headers from purported spammers, because it could be a completely bogus message.
I note, however, that this same problem exists with physical mail. There's nothing that stops someone from writing a letter and putting your company's name to it. It just costs more.
For more information on this topic, visit these other SearchSecurity.com resources:
Best Web Links: Secure Messaging
Tech Tip: E-mail security issues
Executive Security Briefing: Think your e-mail is secure? Think again
This was first published in November 2002