"not_found" status is placed to the cache when the MX is not
responding. SMTP reply 5xx must not be returned at this case.
This behavior discovered in 7.99.92 (git 2012-03-21) but it
could be earlier.

This patch is attempt to disable caching mf_timeout case.

diff -Nur mailfromd-7.99.92-20130730.old/src/savsrv.c mailfromd-7.99.92-20130730/src/savsrv.c
--- mailfromd-7.99.92-20130730.old/src/savsrv.c	2012-02-09 09:57:59.000000000 +0400
+++ mailfromd-7.99.92-20130730/src/savsrv.c	2013-10-22 14:46:12.931146277 +0400
@@ -186,8 +186,10 @@
 				break;
 		}
 		rc = callout_standard(qp->io);
+		if ( rc != mf_timeout ) {
+			cache_insert(smtp_io_email(qp->io), rc);
+		}
 		STATUS_FIXUP(rc);
-		cache_insert(smtp_io_email(qp->io), rc);
 		break;
 
 	case cmode_mx_only:
@@ -212,9 +214,11 @@
 				break;
 		}			
 		rc = callout_strict(qp->io, qp->hostname);
+		if ( rc != mf_timeout ) {
+			cache_insert2(smtp_io_email(qp->io),
+				      qp->hostname, rc);
+		}
 		STATUS_FIXUP(rc);
-		cache_insert2(smtp_io_email(qp->io),
-			      qp->hostname, rc);
 		break;
 
 	case cmode_host_only: