Fix DNS: The DNSSEC resolver queries a single nameserver at a time, working around a hickory-resolver race that cancels the TCP retry when two nameservers return a truncated response in parallel

CHANGELOG.md

@@ -11,6 +11,10 @@ If you are upgrading from v0.16.x, replace the binary (or run `docker pull`). If
 ## Changed
 
 ## Fixed
+- DNS: The DNSSEC resolver queries a single nameserver at a time, working around a `hickory-resolver` race that cancels the TCP retry when two nameservers return a truncated response in parallel.
+- Troubleshoot tool:
+  - MX records are resolved through the DNSSEC-validating resolver, matching the resolver used by the delivery path.
+  - A TLSA lookup that fails or returns bogus records stops the delivery attempt for that host, instead of continuing without DANE.
 - WebUI: A failed update no longer takes an `Application` offline.
 
 ## [0.16.22] - 2026-09-13

crates/common/src/config/smtp/resolver.rs

@@ -214,6 +214,7 @@ impl Resolvers {
         let config_dnssec = resolver_config.clone();
         let mut opts_dnssec = opts.clone();
         opts_dnssec.validate = true;
+        opts_dnssec.num_concurrent_reqs = 1;
 
         let dnssec = DnssecResolver {
             resolver: TokioResolver::builder_with_config(
@@ -343,6 +344,7 @@ impl Default for Resolvers {
         let config_dnssec = config.clone();
         let mut opts_dnssec = opts.clone();
         opts_dnssec.validate = true;
+        opts_dnssec.num_concurrent_reqs = 1;
 
         Self {
             dns: MessageAuthenticator::new(config, opts).expect("Failed to build DNS resolver"),

crates/http/src/api/diagnose.rs

@@ -230,14 +230,7 @@ async fn delivery_diagnose(
 
     // Lookup MX
     let now = Instant::now();
-    let mxs = match server
-        .core
-        .smtp
-        .resolvers
-        .dns
-        .mx_lookup(&domain, Some(&server.inner.cache.dns_mx))
-        .await
-    {
+    let mxs = match server.mx_lookup(domain.as_str()).await {
         Ok(mxs) => mxs,
         Err(err) => {
             tx.send(DeliveryStage::MxLookupError {
@@ -419,7 +412,7 @@ async fn delivery_diagnose(
                 })
                 .await?;
 
-                None
+                continue 'outer;
             }
             Ok(TlsaResult::Missing) => {
                 tx.send(DeliveryStage::TlsaNotFound {
@@ -440,14 +433,17 @@ async fn delivery_diagnose(
                         reason: "No TLSA records found for MX".to_string(),
                     })
                     .await?;
+
+                    None
                 } else {
                     tx.send(DeliveryStage::TlsaLookupError {
                         elapsed: now.elapsed_ms(),
                         reason: err.to_string(),
                     })
                     .await?;
+
+                    continue 'outer;
                 }
-                None
             }
         };