#13: implement Outbound Queue, Retry Backoff Engine & DSN Bounce Generator (RFC 3464)

services/protocols/internal/infrastructure/netdns/mx_resolver.go

@@ -0,0 +1,40 @@
+package netdns
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"sort"
+	"strings"
+)
+
+type NetMXResolver struct{}
+
+func NewNetMXResolver() *NetMXResolver {
+	return &NetMXResolver{}
+}
+
+func (r *NetMXResolver) LookupMX(ctx context.Context, domain string) ([]string, error) {
+	mxs, err := net.DefaultResolver.LookupMX(ctx, domain)
+	if err != nil {
+		return nil, fmt.Errorf("lookup mx for %s: %w", domain, err)
+	}
+
+	sort.Slice(mxs, func(i, j int) bool {
+		return mxs[i].Pref < mxs[j].Pref
+	})
+
+	var hosts []string
+	for _, mx := range mxs {
+		h := strings.TrimSuffix(mx.Host, ".")
+		if h != "" {
+			hosts = append(hosts, h)
+		}
+	}
+
+	if len(hosts) == 0 {
+		hosts = []string{domain}
+	}
+
+	return hosts, nil
+}