email features in platform level 1
this is a meta issue for all the level 1 email platform issues.
level 1 includes:
- spf
- dkim
- rbls
- milter spam blocking
- ability to block a user from sending email
level 2 includes:
- dnssec
- dane
- starttls + dane
(from redmine: created on 2014-07-29, relates #5906, relates #5685, relates #4282, relates #3634 (closed), relates #5647, relates #4283, relates #5072, relates #5053 (closed), relates #4284, relates #4285, relates #7690, relates #7868, relates #7869)