diff --git a/smtpd/utils.go b/smtpd/utils.go
index 6c55ff6..165e76d 100644
--- a/smtpd/utils.go
+++ b/smtpd/utils.go
@@ -20,7 +20,7 @@ func ParseMailboxName(localPart string) (result string, err error) {
 
 	invalid := make([]byte, 0, 10)
 
-	for i := 0; i<len(result); i++ {
+	for i := 0; i < len(result); i++ {
 		c := result[i]
 		switch {
 		case 'a' <= c && c <= 'z':
@@ -73,15 +73,14 @@ func ValidateDomainPart(domain string) bool {
 	}
 	prev := '.'
 	labelLen := 0
-	hasLetters := false
+	hasAlphaNum := false
 
 	for _, c := range domain {
 		switch {
-		case ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') || c == '_':
+		case ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') ||
+			('0' <= c && c <= '9') || c == '_':
 			// Must contain some of these to be a valid label
-			hasLetters = true
-			labelLen++
-		case '0' <= c && c <= '9':
+			hasAlphaNum = true
 			labelLen++
 		case c == '-':
 			if prev == '.' {
@@ -96,11 +95,11 @@ func ValidateDomainPart(domain string) bool {
 			if labelLen > 63 {
 				return false
 			}
-			if !hasLetters {
+			if !hasAlphaNum {
 				return false
 			}
 			labelLen = 0
-			hasLetters = false
+			hasAlphaNum = false
 		default:
 			// Unknown character
 			return false
diff --git a/smtpd/utils_test.go b/smtpd/utils_test.go
index 80fd5f1..7f5e72d 100644
--- a/smtpd/utils_test.go
+++ b/smtpd/utils_test.go
@@ -1,14 +1,15 @@
 package smtpd
 
 import (
-	"github.com/stretchr/testify/assert"
 	"strings"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestParseMailboxName(t *testing.T) {
-	var validTable = []struct{
-		input string
+	var validTable = []struct {
+		input  string
 		expect string
 	}{
 		{"mailbox", "mailbox"},
@@ -33,7 +34,7 @@ func TestParseMailboxName(t *testing.T) {
 		}
 	}
 
-	var invalidTable = []struct{
+	var invalidTable = []struct {
 		input, msg string
 	}{
 		{"", "Empty mailbox name is not permitted"},
@@ -74,9 +75,10 @@ func TestValidateDomain(t *testing.T) {
 		{"_domainkey.foo.com", true, "Underscores are allowed"},
 		{"bar.com.", true, "Must be able to end with a dot"},
 		{"ABC.6DBS.com", true, "Mixed case is OK"},
+		{"mail.123.com", true, "Number only label valid"},
+		{"123.com", true, "Number only label valid"},
 		{"google..com", false, "Double dot not valid"},
 		{".foo.com", false, "Cannot start with a dot"},
-		{"mail.123.com", false, "Number only label not valid"},
 		{"google\r.com", false, "Special chars not allowed"},
 		{"foo.-bar.com", false, "Label cannot start with hyphen"},
 		{"foo-.bar.com", false, "Label cannot end with hyphen"},