dns: resolve ambiguity of null mx and failed host lookups

Tom "Edge Case" Mairs found an issue where what I think happened
was that the DNS records changed since we first established our
own internal data structures.

When the new set of records doesn't resolve to any A or MX entries
we'd return the same empty list of addresses that we used to signal
a null mx and incorrectly fail the remainder of that queue.

This commit introduces a separate state for the null mx state so that
we can instead delay the queue.

crates/dns-resolver/src/lib.rs

@@ -140,13 +140,13 @@ impl MailExchanger {
         Ok(mx)
     }
 
-    pub async fn resolve_addresses(&self) -> Vec<ResolvedAddress> {
+    pub async fn resolve_addresses(&self) -> ResolvedMxAddresses {
         let mut result = vec![];
 
         for mx_host in &self.hosts {
             // '.' is a null mx; skip trying to resolve it
             if mx_host == "." {
-                continue;
+                return ResolvedMxAddresses::NullMx;
             }
 
             // Handle the literal address case
@@ -174,10 +174,16 @@ impl MailExchanger {
             }
         }
         result.reverse();
-        result
+        ResolvedMxAddresses::Addresses(result)
     }
 }
 
+#[derive(Debug, Clone)]
+pub enum ResolvedMxAddresses {
+    NullMx,
+    Addresses(Vec<ResolvedAddress>),
+}
+
 struct ByPreference {
     hosts: Vec<String>,
     pref: u16,
@@ -416,12 +422,14 @@ MailExchanger {
         k9::snapshot!(
             v4_loopback.resolve_addresses().await,
             r#"
-[
-    ResolvedAddress {
-        name: "127.0.0.1",
-        addr: 127.0.0.1,
-    },
-]
+Addresses(
+    [
+        ResolvedAddress {
+            name: "127.0.0.1",
+            addr: 127.0.0.1,
+        },
+    ],
+)
 "#
         );
 
@@ -447,12 +455,14 @@ MailExchanger {
         k9::snapshot!(
             v6_loopback_non_conforming.resolve_addresses().await,
             r#"
-[
-    ResolvedAddress {
-        name: "::1",
-        addr: ::1,
-    },
-]
+Addresses(
+    [
+        ResolvedAddress {
+            name: "::1",
+            addr: ::1,
+        },
+    ],
+)
 "#
         );
 
@@ -478,12 +488,14 @@ MailExchanger {
         k9::snapshot!(
             v6_loopback.resolve_addresses().await,
             r#"
-[
-    ResolvedAddress {
-        name: "::1",
-        addr: ::1,
-    },
-]
+Addresses(
+    [
+        ResolvedAddress {
+            name: "::1",
+            addr: ::1,
+        },
+    ],
+)
 "#
         );
     }

crates/kumod/src/smtp_dispatcher.rs

@@ -7,6 +7,7 @@ use crate::runtime::{rt_spawn, spawn};
 use crate::spool::SpoolManager;
 use anyhow::Context;
 use async_trait::async_trait;
+use dns_resolver::ResolvedMxAddresses;
 use kumo_log_types::ResolvedAddress;
 use message::Message;
 use rfc5321::{ClientError, EnhancedStatusCode, ForwardPath, Response, ReversePath, SmtpClient};
@@ -34,7 +35,7 @@ impl SmtpDispatcher {
             },
         };
 
-        let mut addresses = dispatcher
+        let addresses = dispatcher
             .mx
             .as_ref()
             .expect("to have mx when doing smtp")
@@ -42,16 +43,35 @@ impl SmtpDispatcher {
             .await;
         tracing::trace!("mx resolved to {addresses:?}");
 
+        let mut addresses = match addresses {
+            ResolvedMxAddresses::NullMx => {
+                dispatcher
+                    .bulk_ready_queue_operation(Response {
+                        code: 556,
+                        enhanced_code: Some(EnhancedStatusCode {
+                            class: 5,
+                            subject: 1,
+                            detail: 10,
+                        }),
+                        content: "Recipient address has a null MX".to_string(),
+                        command: None,
+                    })
+                    .await;
+                return Ok(None);
+            }
+            ResolvedMxAddresses::Addresses(a) => a,
+        };
+
         if addresses.is_empty() {
             dispatcher
                 .bulk_ready_queue_operation(Response {
-                    code: 556,
+                    code: 451,
                     enhanced_code: Some(EnhancedStatusCode {
-                        class: 5,
-                        subject: 1,
-                        detail: 10,
+                        class: 4,
+                        subject: 4,
+                        detail: 4,
                     }),
-                    content: "Recipient address has a null MX".to_string(),
+                    content: "MX didn't resolve to any hosts".to_string(),
                     command: None,
                 })
                 .await;