The bootstrap question is addressed by requiring the end-user to know their e-mail address and password. Based on the domain name, the implementation would reach out to https://something.domain-name.tld and download the relevant "schema" and data for IMAP, SMTP, POP3, etc, in ordered priority. Based on what the e-mail client could support, the desired settings would be displayed, and upon end-user approval, applied.
End-user approval? That means support calls, ISPs wouldn't like that. I can believe something like this could be made to work, but I would think hard about all the way that web sessions can get screwed up or hijacked before I persuaded myself that a scheme was likely to work where it needed to work (e.g., when connecting to a hotspot that hijacks all web sessions until you log in) while not being subject to hostile spoofing. Followups definitely to IETF-something. R's, John