You won't find many patches specific to Outlook or Outlook Express. When patches are released, they're usually pacakaged as an Office or Internet Explorer patch, repsectively. I wouldn't necessarily call the problems with either product vulnerabilities. In many cases, the vulnerabilities have more to do with a user's understanding of the product than the product itself (i.e. not disabling preview, launching attachments from unknown senders, etc.).
That said, you'll need to hit the security site mentioned above and search for Office (all versions), Outlook (all versions), Outlook Express (all versions), and Internet Explorer (all versions). Again, you won't find many patches specific to the mail clients. What you'll see are things like, "added switch to enable users to disable HTML previews," and such.
As with any software product, your customers should understand the features available in the product, the risks associated with using those features, and how to mitigate those risks.
Trev