As SPS v1 admin I have a lot of error messages from SMTP server about undeliverable subscriptions notifications There is no way to find any user subscriptions using e-mail address of subscriber. I have done some ASP pages, which may partially eluminate this problem. All codes based on Microsoft WebDAV Sample application from Exchange SDK. Implementation is a simple: create new dasboard in your portal. Create new webpart. Set type of source as HTML. Copy and paste codes provided below.
Change txtSourceURL.Value as your need. Change logon name and password at request initialisation string "oXMLHTTP.Open "SEARCH", strSourceURL, False, "domain\admin", "password"". Save webpart and dashboard content. Voila! It works!. Each ASP required its own dasboard. I have check these code on my portal (SP1) only. It may not works everywere. You can modify codes as you need. Of course, running codes is your own risk. No warranty.
"List of All Known subscriptions for given workspace"
Source code downloadable here:

http://www.spsfaq.com/downloads/ListAllSubs.zip

Edwin

(check all formatting for extra spaces etc, I had to copy this out of the newsgroups)