I thought I saw some discussion on this list about which option an MUA
should choose if it was presented with IMAP, POP3, POP3S, or IMAPS.
While, ultimately this choice is the user's, perhaps a
_mail._tcp.example.com SRV record could be used. The response could
state simply "imaps" or something which would help the MUA pick a
default option. All the other SRV records would be used as suggested
still. It would just help in the auto-configuration of an MUA where a
user doesn't know which option to pick.
This is actually can do more than provide a single default - you could have
multiple records with this prefix and use the priority fields to specify
the preference order for protocol selection.
But even this isn't really necessary to specify precedence among protocols. The
specification could say that pop/pops/imap/imaps records should be considered
as a group, and absent client configuration to the contrary should be used in
the order specified by their individual priority fields. For example, an imaps
record with a higher priority than any imap record would indicate imaps is
preferred over imap.
RFC 2782 appears to allow schemes like this. But whether it's worth defining
such a thing is another matter. My personal opinion is that it's just a bit
outside my comfort zone in terms of complexity, but if others feel strongly
that servers should be able to specify protocol preferences this is a way to do
with it with no additional DNS records or supplementary protocols.
Ned