Based on the debian patch:
http://patch-tracker.debian.org/patch/series/view/spamass-milter/0.3.1-10/10_dont_handle_authenticated_users.diff

--- spamass-milter.cpp	2011-06-08 11:56:33.000000000 +0000
+++ spamass-milter.cpp	2011-06-08 12:04:41.000000000 +0000
@@ -170,6 +170,7 @@
 bool flag_full_email = false;		/* pass full email address to spamc */
 bool flag_expand = false;	/* alias/virtusertable expansion */
 bool warnedmacro = false;	/* have we logged that we couldn't fetch a macro? */
+bool ignore_authenticated_senders = false;
 
 #if defined(__FreeBSD__) /* popen bug - see PR bin/50770 */
 static pthread_mutex_t popen_mutex = PTHREAD_MUTEX_INITIALIZER;
@@ -181,7 +182,7 @@
 main(int argc, char* argv[])
 {
    int c, err = 0;
-   const char *args = "fd:mMp:P:r:u:D:i:b:B:e:x";
+   const char *args = "fd:mMp:P:r:u:D:i:Ib:B:e:x";
    char *sock = NULL;
    bool dofork = false;
    char *pidfilename = NULL;
@@ -213,6 +214,10 @@
 				debug(D_MISC, "Parsing ignore list");
 				parse_networklist(optarg, &ignorenets);
 				break;
+			case 'I':
+				debug(D_MISC, "Ignore authenticated senders");
+				ignore_authenticated_senders = true;
+				break;
 			case 'm':
 				dontmodifyspam = true;
 				smfilter.xxfi_flags &= ~SMFIF_CHGBODY;
@@ -280,7 +285,7 @@
       cout << PACKAGE_NAME << " - Version " << PACKAGE_VERSION << endl;
       cout << "SpamAssassin Sendmail Milter Plugin" << endl;
       cout << "Usage: spamass-milter -p socket [-b|-B bucket] [-d xx[,yy...]] [-D host]" << endl;
-      cout << "                      [-e defaultdomain] [-f] [-i networks] [-m] [-M]" << endl;
+      cout << "                      [-e defaultdomain] [-f] [-i networks] [-I] [-m] [-M]" << endl;
       cout << "                      [-P pidfile] [-r nn] [-u defaultuser] [-x]" << endl;
       cout << "                      [-- spamc args ]" << endl;
       cout << "   -p socket: path to create socket" << endl;
@@ -294,6 +299,7 @@
       cout << "   -f: fork into background" << endl;
       cout << "   -i: skip (ignore) checks from these IPs or netblocks" << endl;
       cout << "          example: -i 192.168.12.5,10.0.0.0/8,172.16.0.0/255.255.0.0" << endl;
+      cout << "   -I: skip (ignore) checks if sender is authenticated" << endl;
       cout << "   -m: don't modify body, Content-type: or Subject:" << endl;
       cout << "   -M: don't modify the message at all" << endl;
       cout << "   -P pidfile: Put processid in pidfile" << endl;
@@ -783,6 +789,22 @@
   }
   /* debug(D_ALWAYS, "ZZZ got private context %p", sctx); */
 
+  if (ignore_authenticated_senders)
+  {
+    char *auth_authen;
+
+    auth_authen = smfi_getsymval(ctx, "{auth_authen}");
+    debug(D_MISC, "auth_authen=%s", auth_authen ?: "<unauthenticated>");
+
+    if (auth_authen)
+    {
+      debug(D_MISC, "sender authenticated (%s) - accepting message",
+          auth_authen);
+      debug(D_FUNC, "mlfi_envfrom: exit ignore");
+      return SMFIS_ACCEPT;
+    }
+  }
+
   debug(D_FUNC, "mlfi_envfrom: enter");
   try {
     // launch new SpamAssassin
--- spamass-milter.1.in	2011-06-08 12:05:35.000000000 +0000
+++ spamass-milter.1.in	2011-06-08 12:06:57.000000000 +0000
@@ -14,6 +14,7 @@
 .Op Fl e Ar defaultdomain
 .Op Fl f
 .Op Fl i Ar networks
+.Op Fl I
 .Op Fl m
 .Op Fl M
 .Op Fl P Ar pidfile
@@ -119,6 +120,8 @@
 flags will append to the list.
 For example, if you list all your internal networks, no outgoing emails
 will be filtered.
+.It Fl I
+Ignores messages if the sender has authenticated via SMTP AUTH.
 .It Fl m
 Disables modification of the 
 .Ql Subject: