admin check: do not raise error when forward-confirmed reverse dns do…

…es not match hostname

this should be relatively common with setups involving NAT.
so we do warn about it when NAT isn't active since it could highlight potential
misconfiguration.

for issue #239 by exander77

webadmin/admin.go

@@ -665,7 +665,7 @@ EOF
 				continue
 			}
 			if len(addrs) != 1 {
-				addf(&r.IPRev.Errors, "Expected exactly 1 name for %s of %s, got %d (%v)", ip, host, len(addrs), addrs)
+				addf(&r.IPRev.Warnings, "Expected exactly 1 name for %s of %s, got %d (%v)", ip, host, len(addrs), addrs)
 			}
 			var match bool
 			for i, a := range addrs {
@@ -679,8 +679,8 @@ EOF
 					match = true
 				}
 			}
-			if !match {
-				addf(&r.IPRev.Errors, "Reverse name(s) %s for ip %s do not match hostname %s, which will cause other mail servers to reject incoming messages from this IP.", strings.Join(addrs, ","), ip, host)
+			if !match && !isNAT && host == mox.Conf.Static.HostnameDomain {
+				addf(&r.IPRev.Warnings, "IP %s with name(s) %s is forward confirmed, but does not match hostname %s.", ip, strings.Join(addrs, ","), host)
 			}
 			r.IPRev.IPNames[ip] = addrs
 		}