fix system generated bug

internal/syntax/user.go

@@ -120,6 +120,7 @@ func hasMultipleNumericSegments(username string) bool {
 func isCommonNamePattern(username string) bool {
 	namePatterns := []*regexp.Regexp{
 		regexp.MustCompile(`^[a-z]+\.[a-z]+\d{0,4}$`),  // john.doe123
+		regexp.MustCompile(`^[a-z]+\_[a-z]+\d{0,4}$`),  // john.doe123
 		regexp.MustCompile(`^[a-z]+\.[a-z]\.[a-z]+$`),  // john.m.doe
 		regexp.MustCompile(`^[a-z]+\.[a-z]+\.[a-z]+$`), // john.michael.doe
 		regexp.MustCompile(`^[a-z]+[a-z0-9]{0,4}$`),    // johndoe, john123

mailvalidate/email_syntax_test.go

@@ -3,8 +3,9 @@ package mailvalidate_test
 import (
 	"testing"
 
-	"github.com/customeros/mailsherpa/mailvalidate"
 	"github.com/stretchr/testify/assert"
+
+	"github.com/customeros/mailsherpa/mailvalidate"
 )
 
 func TestValidateEmailSyntax(t *testing.T) {
@@ -80,6 +81,21 @@ func TestValidateEmailSyntax(t *testing.T) {
 			expected:    mailvalidate.SyntaxValidation{},
 			description: "Invalid email should return empty validation struct",
 		},
+		{
+			name:  "Underscore Username",
+			email: "bob_smith@google.com",
+			expected: mailvalidate.SyntaxValidation{
+				IsValid:           true,
+				User:              "bob_smith",
+				Domain:            "google.com",
+				CleanEmail:        "bob_smith@google.com",
+				IsRoleAccount:     false,
+				IsFreeAccount:     false,
+				IsSystemGenerated: false,
+				Error:             "",
+			},
+			description: "Underscores in username should be supported",
+		},
 		{
 			name:  "Mixed Case with Emoji Email",
 			email: "Rob.Name😆@Gmail.com",