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 Exte

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 :)