I've never understood why contacts can't be stored inside a mail folder, so that every computer you setup thunderbird on gets its address book synced..
We do have a standard for this. RFC 6186 "Use of SRV Records for Locating Email Submission/Access Services" and RFC 6764 "Locating Services for Calendaring Extensions to WebDAV (CalDAV) and vCard Extensions to WebDAV (CardDAV)" should be sufficient.
Support these and it should be possible for any client email app to automatically configure both the email account and address book syncing using existing standard protocols. No need for a new one :)
That actually doesn't sound like a terrible idea. Structured markup in the email could be parsed by a client that knew what it was, and other clients would just have a folder of emails with contact info in them.
