Mark, Thank you very much for your helpful answers. Your assumptions about my scenario (and my typos) were correct. One follow-up clarification in re: permanently tracking edited posts. My requirements include interoperability with other MUAs: I am creating a publicly-readable presentation of a mailbox that other MUA(s) will be manipulating the purpose of message creation and management. Specifically, a weblog-style presentation of public folder(s) which user(s) manage via Outlook, Outlook Express, or other IMAP MUAs.
So my comment about DELETE-EXPUNGE and using timestamp or timestamp+sender as the basis of permanent links was based on the fact that the Outlook/Exchange model of editability does not preserve both copies, so that option for tracking edits is not available to me. And since my system is not creating the original or edited content, "using a thread id or something similar in the headers" (as Pete Maclean suggested in the "Why is a message immutable?" thread) is not possible. Similarly, unless I'm missing something, searching for Message-ID as you suggested is not possible in this situation: the original message is gone, the edited message of course has a new Message-ID, and Outlook/Exchange does not preserve the original Message-ID anywhere in the edited message header or body. So since I want to create a link that will retrieve the content per the editability model provided by Outlook I don't see any way to address the problem other than by using timestamp (or, more likely, timestamp+sender). Since the Outlook model of editability includes the ability to edit the subject even that is not a suitable part of the permanent key. Of course it may be (perhaps is likely) that Outlook/Exchange provides some internal ID mechanism which persists across such content revision, but not only is this not accessible to me, but I am also creating a presentation layer which will not be limited to interoperability with Outlook/Exchange. If you (or anyone) has other ideas on how to implement permanent IMAP URLs that will work with (e.g.) Outlook's editability model I would love to hear them. Of course if timestamp+sender information is not seen as a suitable mechanism to implement permanent IMAP URLs in an MUA-interoperable way, in the face of a user model that potentially includes message editability, and if there is no other IMAP mechanism for doing so, then perhaps there is merit in an IMAP extension that provided indexability at the UID level as John Milan recently proposed. --Bill McCoy
