diff --git a/pyas2lib/as2.py b/pyas2lib/as2.py
index a81dee1..8173f5d 100644
--- a/pyas2lib/as2.py
+++ b/pyas2lib/as2.py
@@ -288,7 +288,8 @@ def headers_str(self):
         return message_header.encode('utf-8')
 
     def build(self, data, filename=None, subject='AS2 Message',
-              content_type='application/edi-consent', additional_headers=None):
+              content_type='application/edi-consent', additional_headers=None,
+              disposition_notification_to='no-reply@pyas2.com'):
 
         """Function builds the AS2 message. Compresses, signs and encrypts
         the payload if applicable.
@@ -309,6 +310,10 @@ def build(self, data, filename=None, subject='AS2 Message',
         :param additional_headers:
             Any additional headers to be included as part of the AS2 message.
 
+        :param disposition_notification_to:
+            Email address for disposition-notification-to header entry.
+            (default "no-reply@pyas2.com")
+
         """
 
         # Validations
@@ -413,7 +418,7 @@ def build(self, data, filename=None, subject='AS2 Message',
                 f'Encrypted message {self.message_id} payload as:\n{self.payload.as_string()}')
 
         if self.receiver.mdn_mode:
-            as2_headers['disposition-notification-to'] = 'no-reply@pyas2.com'
+            as2_headers['disposition-notification-to'] = disposition_notification_to
             if self.receiver.mdn_digest_alg:
                 as2_headers['disposition-notification-options'] = \
                     f'signed-receipt-protocol=required, pkcs7-signature; ' \
diff --git a/pyas2lib/tests/test_advanced.py b/pyas2lib/tests/test_advanced.py
index 8417051..fd15f24 100644
--- a/pyas2lib/tests/test_advanced.py
+++ b/pyas2lib/tests/test_advanced.py
@@ -457,8 +457,7 @@ def xtest_process_message(self):
     def test_process_mdn(self):
         """ Test processing mdn received from Sterling Integrator"""
         msg = as2.Message(sender=self.org, receiver=self.partner)
-        msg.message_id = '151694007918.24690.7052273208458909245@' \
-                             'ip-172-31-14-209.ec2.internal'
+        msg.message_id = '151694007918.24690.7052273208458909245@ip-172-31-14-209.ec2.internal'
 
         as2mdn = as2.Mdn()
         # Parse the mdn and get the message status
diff --git a/pyas2lib/tests/test_utils.py b/pyas2lib/tests/test_utils.py
index c14a246..8222af2 100644
--- a/pyas2lib/tests/test_utils.py
+++ b/pyas2lib/tests/test_utils.py
@@ -52,12 +52,10 @@ def test_extract_certificate_info():
      in PEM or DER format"""
 
     cert_info = {
-        'valid_from': datetime.datetime(2019, 6, 3, 11, 32, 57),
-        'valid_to': datetime.datetime(2029, 5, 31, 11, 32, 57),
-        'subject': [('C', 'AU'), ('ST', 'Some-State'),
-                    ('O', 'pyas2lib'), ('CN', 'test')],
-        'issuer': [('C', 'AU'), ('ST', 'Some-State'),
-                   ('O', 'pyas2lib'), ('CN', 'test')],
+        'valid_from': datetime.datetime(2019, 6, 3, 11, 32, 57, tzinfo=datetime.timezone.utc),
+        'valid_to': datetime.datetime(2029, 5, 31, 11, 32, 57, tzinfo=datetime.timezone.utc),
+        'subject': [('C', 'AU'), ('ST', 'Some-State'), ('O', 'pyas2lib'), ('CN', 'test')],
+        'issuer': [('C', 'AU'), ('ST', 'Some-State'), ('O', 'pyas2lib'), ('CN', 'test')],
         'serial': 13747137503594840569
     }
     cert_empty = {
diff --git a/pyas2lib/utils.py b/pyas2lib/utils.py
index 073747c..72b33f0 100644
--- a/pyas2lib/utils.py
+++ b/pyas2lib/utils.py
@@ -10,7 +10,7 @@
 from io import BytesIO
 
 from pyas2lib.exceptions import AS2Exception
-from datetime import datetime
+from datetime import datetime, timezone
 
 
 def unquote_as2name(quoted_name: str):
@@ -205,8 +205,8 @@ def extract_certificate_info(cert: bytes):
 
     :param cert: the certificate as byte string in PEM or DER format
     :return: a dictionary holding certificate information:
-                valid_from (datetime)
-                valid_to (datetime)
+                valid_from (datetime) - UTC
+                valid_to (datetime) - UTC
                 subject (list of name, value tuples)
                 issuer (list of name, value tuples)
                 serial (int)
@@ -233,9 +233,11 @@ def extract_certificate_info(cert: bytes):
 
             # on successful load, extract the various fields into the dictionary
             cert_info['valid_from'] = datetime.strptime(
-                certificate.get_notBefore().decode('utf8'), "%Y%m%d%H%M%SZ")
+                certificate.get_notBefore().decode('utf8'), "%Y%m%d%H%M%SZ").\
+                replace(tzinfo=timezone.utc)
             cert_info['valid_to'] = datetime.strptime(
-                certificate.get_notAfter().decode('utf8'), "%Y%m%d%H%M%SZ")
+                certificate.get_notAfter().decode('utf8'), "%Y%m%d%H%M%SZ").\
+                replace(tzinfo=timezone.utc)
             cert_info['subject'] = [
                 tuple(item.decode('utf8') for item in sets)
                 for sets in certificate.get_subject().get_components()]