Marking a mail as junk simply redirects it to the junk box. Helps reduce crap the user sees but doesn't actually change the landscape. Overtime, junk folder traffic may exceed inbox. My 10+ years old Hotmail and yahoo accounts are already there - daily incoming rate to junk folder is higher than my inbox. If Exchange could reject (not bounce) mails from explicitly junked senders as invalid email ID (511?), I suspect ...more »
Currently neither native to Exchange so a third party MTA on-prem or in cloud is required. I've had some customers that required this, not having this native added additional cost.
More info: http://dkim.org/
Exchange 2007/2010 supported the Connection Filtering Agent on the Hub Transport role. This agent is not available on the Exchange 2013 CAS and Mailbox roles.
This means we can't use real-time block lists on Exchange 2013 servers. Which is a shame because RBL's have proven to be a great measurement against spam.