Both Jotspot and MediaWiki allow this. The latter supports many plugins, of which this is one. The format is essentially PageTitle @something. Jotspot is a hosted solution where you get your own email address, MediaWiki is self-contained, and you provide it with an inbox to monitor your inbox.
Articles are added to pages if they already exist, or a new page is created if it is not. This requires some discipline to indicate conventions, but is great for CC'ing.
We use MediaWiki here, and I really like it. It has the same disadvantages as many other Wiki packages (for example, it is difficult to reorganize without orphaned pages), but it is also good, if not better, than other Wiki packages that I used.
Andrew Grant
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