[好的，我会将此作为新答案发布，以保持此答案的清洁而不删除所有先前发布的内容.]

[OK, I'll post this as a new answer to keep this one clean without deleting all of the previously posted content.]

我刚刚重复了我另一个答案中提到的研究，似乎在过去三年中情况有了很大的改善.Outlook 2003 (SP3) 和 Outlook 2007 (SP2) 现在都会自动识别 MS 产品支持邮件中列出的所有协议，该邮件引用自我的旧答案链接的 NG 线程，包括 url:-协议！

I have just repeated the research mentioned in my other answer and it seems that things have greatly improved in the last three years. Both Outlook 2003 (SP3) and Outlook 2007 (SP2) now automatically recognize all of the protocols listed in the MS product support mail quoted in the NG thread linked from my older answer, including the url:-protocol!

后者意味着您现在可以编写如下内容:

The latter means that you can now write something like:

url:irma:whatever

并正确链接和处理它.然而，Outlook 将在执行协议处理程序之前显示一个安全确认对话框.

and have it properly linkified and handled. Outlook will however display a security confirmation dialog before executing the protocol handler.

更新:请注意，在 Outlook 2003 SP3 中，新协议(例如 url:、mms:// 等)仅适用于在 HTML 邮件中自动链接.纯文本检查器的行为如我的其他答案中所述.然而，它确实适用于 Outlook 2007 SP2 中的纯文本邮件.

Update: Note that in Outlook 2003 SP3 the new protocols (e.g. url:, mms://, etc.) are only auto-linkified in HTML-mails. The plain-text inspector behaves as described in my other answer. It does work with plain-text mails in Outlook 2007 SP2, however.

我仍然没有发现可以在 Outlook 中简单地注册自定义协议的能力，这些协议无需 url:-前缀即可工作.

I have still found no hints at the ability to simply register custom protocols within Outlook that would work without the url:-prefix, however.