Update to default exit policy

[krishna e bera]

I'm not clear on how authentication (on any port) stops spam,
other than the ISP cutting off a given userid after complaints.
A lot of spam already comes from malware infected computers 
via their legitimately configured email.
Those computers are probably not using Tor, let alone transparent proxy, 
but malware could grab their credentials and then 
use Tor on another host to send out spam over port 587,
if that port was allowed in exit policies.