we've been having problems with nulls showing
up in email.  it turns out that the dot destuffing
in smtpd is the culprit.  i also added some extra
paranoid stuff to the getnl() stuff to make the
sender isn't messing with us.  to trigger this bug
send a mail with a dot on a line by itself. (or
telnet to smtpd and forge the email yourself,
this time doing the dot-stuffing by hand.)

Reference: /n/sources/patch/applied/smtpdnull
Date: Mon Oct  6 21:29:03 CES 2008
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/upas/smtp/smtpd.c	Mon Oct  6 21:28:58 2008
+++ /sys/src/cmd/upas/smtp/smtpd.c	Mon Oct  6 21:28:52 2008
@@ -718,6 +718,8 @@
 			fprint(2, "%c", c);
 		}
 		switch(c){
+		case 0:
+			break;
 		case -1:
 			goto out;
 		case '\r':
@@ -1118,8 +1120,12 @@
 			sawdot = 1;
 			break;
 		}
+		if(cp[0] == '.'){
+			cp++;
+			n--;
+		}
 		nbytes += n;
-		if(status == 0 && Bwrite(pp->std[0]->fp, *cp == '.' ? cp+1 : cp, n) < 0){
+		if(status == 0 && Bwrite(pp->std[0]->fp, cp, n) < 0){
 			piperror = "write error 3";
 			status = 1;
 		}