mox!

smtpserver/mx.go

@@ -0,0 +1,37 @@
+package smtpserver
+
+import (
+	"context"
+	"net"
+
+	"github.com/mjl-/mox/dns"
+)
+
+// checks if domain can accept email.
+// i.e. if it has no null mx record, regular mx records or resolve to an address.
+func checkMXRecords(ctx context.Context, resolver dns.Resolver, d dns.Domain) (bool, error) {
+	// Note: LookupMX can return an error and still return records.
+	mx, err := resolver.LookupMX(ctx, d.ASCII+".")
+	if err == nil && len(mx) == 1 && mx[0].Host == "." {
+		// Null MX record, explicit signal that remote does not accept email.
+		return false, nil
+	}
+	// Treat all errors that are not "no mx record" as temporary. E.g. timeout, malformed record, remote server error.
+	if err != nil && !dns.IsNotFound(err) {
+		return false, err
+	}
+	if len(mx) == 0 {
+		mx = []*net.MX{{Host: d.ASCII + "."}}
+	}
+	var lastErr error
+	for _, x := range mx {
+		ips, err := resolver.LookupIPAddr(ctx, x.Host)
+		if len(ips) > 0 {
+			return true, nil
+		}
+		if err != nil && !dns.IsNotFound(err) {
+			lastErr = err
+		}
+	}
+	return false, lastErr
+}