[9fans] fake bounce spam

[erik quanstrom <quanstro@quanstro.net>]

starting saturday, i've been getting an increasing amount of
fake bounce spam predending to be a undelivered mail
notice from MAILER-DAEMON or similar.  overnight i
received >500 such emails.  as an emergency measure, i applied
this terrible hack to smtpd:

; 9diff smtpd.c
/n/sources/plan9//sys/src/cmd/upas/smtp/smtpd.c:1175,1180 - smtpd.c:1175,1184
  				"recipients");
  			return -1;
  		}
+ 		if(1){
+ 			werrstr("rejected: /dev/null likely spam ");
+ 			return -1;
+ 		}
  		return 0;
  	}

unfortunately, if i really do send mail that bounces, i won't know
about it.

i suppose it would be much better to accept bounces only from addresses to which
email has been sent.  though that may mean maintaining a list (per user?).

- erik