diff --git a/authheaders/__init__.py b/authheaders/__init__.py
index 37a2c00..4dd0330 100644
--- a/authheaders/__init__.py
+++ b/authheaders/__init__.py
@@ -45,12 +45,8 @@
 
 def get_domain_part(address):
     '''Return domain part of an email address'''
-    if sys.version_info < (3, 0) and isinstance(address, str):
-        address = bytes(address)
-    elif isinstance(address, str):
-        address = bytes(address, 'utf8')
-    res = re.findall(b'@([a-z0-9.]+)', address, re.IGNORECASE)
-    return res[0].lower().decode('ascii')
+    res = re.findall(r'@([-a-z0-9.]+)', address, re.IGNORECASE)
+    return res[-1].lower()
 
 
 def check_psddmarc_list(psdname, dnsfunc=dns_query):
diff --git a/authheaders/test/test_authentication.py b/authheaders/test/test_authentication.py
index 486b4c6..c981a8f 100755
--- a/authheaders/test/test_authentication.py
+++ b/authheaders/test/test_authentication.py
@@ -151,7 +151,14 @@ def test_prev(self):
         self.assertEqual(res, "Authentication-Results: example.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass header.d=example.com header.i=@example.com")
 
     def test_get_domain_part(self):
-        froms_to_test = [['test@example.com', 'example.com'], [""""Test, User" <test@example.com>""", 'example.com'], ["""Test User <test@sub2.example.biz>""", 'sub2.example.biz'], ["""=?UTF-8?B?QmVkIEJhdGggJiBCZXlvbmQ=?=<BdBth&Byond@example.com>""", 'example.com'], ]
+        froms_to_test = [
+            ['test@example.com', 'example.com'],
+            ['test@UPPER.hyphen-minus.example.com', 'upper.hyphen-minus.example.com'],
+            [""""Test, User" <test@example.com>""", 'example.com'],
+            [""""Test@User display name" <test@example.com>""", 'example.com'],
+            ["""Test User <test@sub2.example.biz>""", 'sub2.example.biz'],
+            ["""=?UTF-8?B?QmVkIEJhdGggJiBCZXlvbmQ=?=<BdBth&Byond@example.com>""", 'example.com'],
+        ]
         for body_from in froms_to_test:
             res = get_domain_part(body_from[0])
             self.assertEqual(res, body_from[1])