diff --git a/README.md b/README.md
index 04a3e4d..82016d3 100644
--- a/README.md
+++ b/README.md
@@ -62,7 +62,7 @@ msg.recipients = ['you@example.com', 'me@example.com']
 msg.add_recipient('otherperson@example.com')
 ```
 
-If you have set MAIL_DEFAULT_SENDER you don’t need to set the message sender explicitly, as it will use this configuration value by default:
+If you have set `MAIL_DEFAULT_SENDER` you don’t need to set the message sender explicitly, as it will use this configuration value by default:
 
 ```
 msg = Message('Hello',
@@ -85,7 +85,7 @@ msg.body = 'message body'
 msg.html = '<b>Hello apistar_mail!</b>'
 ```
 
-## Testing
+### Testing
 
 To run the test suite with coverage first install the package in editable mode with it's testing requirements:
 
diff --git a/tests/test_apistar_mail.py b/tests/test_apistar_mail.py
index f70ec3f..4142ea5 100644
--- a/tests/test_apistar_mail.py
+++ b/tests/test_apistar_mail.py
@@ -1,4 +1,8 @@
-from apistar_mail.mail import Message, Mail, force_text
+import base64
+import email
+import re
+import time
+from apistar_mail.mail import Message, Mail, force_text, sanitize_address
 from apistar_mail.exc import MailUnicodeDecodeError
 
 import pytest
@@ -15,6 +19,7 @@ settings = {
     }
 }
 
+# Message
 
 def test_message_init():
     msg = Message(subject="subject",
@@ -49,4 +54,333 @@ def test_raise_unicode_decode_error():
     value = b'\xe5\x93\x88\xe5\x93\x88'
     with pytest.raises(MailUnicodeDecodeError) as excinfo:
         force_text(value, encoding='ascii')
-    assert 'You passed in' in str(excinfo)
\ No newline at end of file
+    assert 'You passed in' in str(excinfo)
+
+
+def test_esmtp_options_properly_initialized():
+    msg = Message(subject='subject')
+    assert msg.mail_options == []
+    assert msg.rcpt_options == []
+
+
+def test_sender_as_tuple():
+    msg = Message(subject='test',
+                  sender=('Me', 'me@example.com'))
+    assert msg.sender == 'Me <me@example.com>'
+
+
+def test_emails_are_sanitized():
+    msg = Message(subject="testing",
+                  sender="sender\r\n@example.com",
+                  reply_to="reply_to\r\n@example.com",
+                  recipients=["recipient\r\n@example.com"])
+    assert 'sender@example.com' in msg.as_string()
+    assert 'reply_to@example.com' in msg.as_string()
+    assert 'recipient@example.com' in msg.as_string()
+
+
+def test_plain_message():
+    plain_text = "Hello Joe,\nHow are you?"
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["to@example.com"],
+                  body=plain_text)
+    assert plain_text == msg.body
+    assert 'Content-Type: text/plain' in msg.as_string()
+
+
+def test_message_str():
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["to@example.com"],
+                  body="some plain text")
+    assert msg.as_string() == str(msg)
+
+
+def test_plain_message_with_attachments():
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["to@example.com"],
+                  body="hello")
+
+    msg.attach(data=b"this is a test",
+               content_type="text/plain")
+
+    assert 'Content-Type: multipart/mixed' in msg.as_string()
+
+
+def test_plain_message_with_ascii_attachment():
+    msg = Message(subject="subject",
+                  recipients=["to@example.com"],
+                  body="hello",
+                  sender="me@example.com", )
+
+    msg.attach(data=b"this is a test",
+               content_type="text/plain",
+               filename='test doc.txt')
+
+    assert 'Content-Disposition: attachment; filename="test doc.txt"' in msg.as_string()
+
+
+def test_plain_message_with_unicode_attachment():
+    msg = Message(subject="subject",
+                  recipients=["to@example.com"],
+                  body="hello",
+                  sender="me@example.com", )
+
+    msg.attach(data=b"this is a test",
+               content_type="text/plain",
+               filename='ünicöde ←→ ✓.txt')
+
+    parsed = email.message_from_string(msg.as_string())
+
+    assert re.sub(r'\s+', ' ', parsed.get_payload()[1].get('Content-Disposition')) in [
+        'attachment; filename*="UTF8\'\'%C3%BCnic%C3%B6de%20%E2%86%90%E2%86%92%20%E2%9C%93.txt"',
+        'attachment; filename*=UTF8\'\'%C3%BCnic%C3%B6de%20%E2%86%90%E2%86%92%20%E2%9C%93.txt'
+    ]
+
+
+def test_plain_message_with_ascii_converted_attachment():
+    msg = Message(subject="subject",
+                  recipients=["to@example.com"],
+                  body="hello",
+                  sender="me@example.com",
+                  ascii_attachments=True)
+
+    msg.attach(data=b"this is a test",
+               content_type="text/plain",
+               filename='ünicödeß ←.→ ✓.txt')
+
+    assert 'Content-Disposition: attachment; filename="unicode . .txt"' in msg.as_string()
+
+
+def test_html_message():
+    html_text = "<p>Hello World</p>"
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["to@example.com"],
+                  html=html_text)
+
+    assert html_text == msg.html
+    assert 'Content-Type: multipart/alternative' in msg.as_string()
+
+
+def test_json_message():
+    json_text = '{"msg": "Hello World!}'
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["to@example.com"],
+                  alts={'json': json_text})
+
+    assert json_text == msg.alts['json']
+    assert 'Content-Type: multipart/alternative' in msg.as_string()
+
+
+def test_html_message_with_attachments():
+    html_text = "<p>Hello World</p>"
+    plain_text = 'Hello World'
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["to@example.com"],
+                  body=plain_text,
+                  html=html_text)
+    msg.attach(data=b"this is a test",
+               content_type="text/plain")
+
+    assert html_text == msg.html
+    assert 'Content-Type: multipart/alternative' in msg.as_string()
+
+    parsed = email.message_from_string(msg.as_string())
+    assert len(parsed.get_payload()) == 2
+
+    body, attachment = parsed.get_payload()
+    assert len(body.get_payload()) == 2
+
+    plain, html = body.get_payload()
+    assert plain.get_payload() == plain_text
+    assert html.get_payload() == html_text
+    assert base64.b64decode(attachment.get_payload()), b'this is a test'
+
+
+def test_date_header():
+    before = time.time()
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["to@example.com"],
+                  body="hello",
+                  date=time.time())
+    after = time.time()
+
+    assert before <= msg.date <= after
+    date_formatted = email.utils.formatdate(msg.date, localtime=True)
+    assert 'Date: ' + date_formatted in msg.as_string()
+
+
+def test_msgid_header():
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["to@example.com"],
+                  body="hello")
+
+    # see RFC 5322 section 3.6.4. for the exact format specification
+    r = re.compile(r"<\S+@\S+>").match(msg.msgId)
+    assert r is not None
+    assert 'Message-ID: ' + msg.msgId in msg.as_string()
+
+
+def test_unicode_sender_tuple():
+    msg = Message(subject="subject",
+                  sender=("ÄÜÖ → ✓", 'from@example.com>'),
+                  recipients=["to@example.com"])
+
+    assert 'From: =?utf-8?b?w4TDnMOWIOKGkiDinJM=?= <from@example.com>' in msg.as_string()
+
+
+def test_unicode_sender():
+    msg = Message(subject="subject",
+                  sender='ÄÜÖ → ✓ <from@example.com>>',
+                  recipients=["to@example.com"])
+
+    assert 'From: =?utf-8?b?w4TDnMOWIOKGkiDinJM=?= <from@example.com>' in msg.as_string()
+
+
+def test_unicode_headers():
+    msg = Message(subject="subject",
+                  sender='ÄÜÖ → ✓ <from@example.com>',
+                  recipients=["Ä <t1@example.com>", "Ü <t2@example.com>"],
+                  cc=["Ö <cc@example.com>"])
+
+    response = msg.as_string()
+    a1 = sanitize_address("Ä <t1@example.com>")
+    a2 = sanitize_address("Ü <t2@example.com>")
+    h1_a = email.header.Header("To: %s, %s" % (a1, a2))
+    h1_b = email.header.Header("To: %s, %s" % (a2, a1))
+    h2 = email.header.Header("From: %s" % sanitize_address("ÄÜÖ → ✓ <from@example.com>"))
+    h3 = email.header.Header("Cc: %s" % sanitize_address("Ö <cc@example.com>"))
+
+    # Ugly, but there's no guaranteed order of the recipients in the header
+    try:
+        assert h1_a.encode() in response
+    except AssertionError:
+        assert h1_b.encode() in response
+
+    assert h2.encode() in response
+    assert h3.encode() in response
+
+
+def test_unicode_subject():
+    msg = Message(subject="sübject",
+                  sender='from@example.com',
+                  recipients=["to@example.com"])
+    assert '=?utf-8?q?s=C3=BCbject?=' in msg.as_string()
+
+
+def test_extra_headers():
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["to@example.com"],
+                  body="hello",
+                  extra_headers={'X-Extra-Header': 'Yes'})
+    assert 'X-Extra-Header: Yes' in  msg.as_string()
+
+
+def test_message_charset():
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["foo@bar.com"],
+                  charset='us-ascii')
+
+    # ascii body
+    msg.body = "normal ascii text"
+    assert 'Content-Type: text/plain; charset="us-ascii"' in msg.as_string()
+
+    # ascii html
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["foo@bar.com"],
+                  charset='us-ascii')
+    msg.body = None
+    msg.html = "<html><h1>hello</h1></html>"
+    assert 'Content-Type: text/html; charset="us-ascii"' in msg.as_string()
+
+    # unicode body
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["foo@bar.com"])
+    msg.body = "ünicöde ←→ ✓"
+    assert 'Content-Type: text/plain; charset="utf-8"' in msg.as_string()
+
+    # unicode body and unicode html
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["foo@bar.com"])
+    msg.html = "ünicöde ←→ ✓"
+    assert 'Content-Type: text/plain; charset="utf-8"' in msg.as_string()
+    assert 'Content-Type: text/html; charset="utf-8"' in msg.as_string()
+
+    # unicode body and attachments
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["foo@bar.com"])
+    msg.html = None
+    msg.attach(data=b"foobar", content_type='text/csv')
+    assert 'Content-Type: text/plain; charset="utf-8"' in msg.as_string()
+
+    # unicode sender as tuple
+    msg = Message(sender=("送信者", "from@example.com"),
+                  subject="表題",
+                  recipients=["foo@bar.com"],
+                  reply_to="返信先 <somebody@example.com>",
+                  charset='shift_jis')  # japanese
+    msg.body = '内容'
+    assert 'From: =?iso-2022-jp?' in msg.as_string()
+    assert 'From: =?utf-8?' not in msg.as_string()
+    assert 'Subject: =?iso-2022-jp?' in msg.as_string()
+    assert 'Subject: =?utf-8?' not in msg.as_string()
+    assert 'Reply-To: =?iso-2022-jp?' in msg.as_string()
+    assert 'Reply-To: =?utf-8?' not in msg.as_string()
+    assert 'Content-Type: text/plain; charset="iso-2022-jp"' in msg.as_string()
+
+    # unicode subject sjis
+    msg = Message(sender="from@example.com",
+                  subject="表題",
+                  recipients=["foo@bar.com"],
+                  charset='shift_jis')  # japanese
+    msg.body = '内容'
+    assert 'Subject: =?iso-2022-jp?' in msg.as_string()
+    assert 'Content-Type: text/plain; charset="iso-2022-jp"', msg.as_string()
+
+    # unicode subject utf-8
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["foo@bar.com"],
+                  charset='utf-8')
+    msg.body = '内容'
+    assert 'Subject: subject' in msg.as_string()
+    assert 'Content-Type: text/plain; charset="utf-8"' in msg.as_string()
+
+    # ascii subject
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["foo@bar.com"],
+                  charset='us-ascii')
+    msg.body = "normal ascii text"
+    assert 'Subject: =?us-ascii?' not in msg.as_string()
+    assert 'Content-Type: text/plain; charset="us-ascii"' in msg.as_string()
+
+    # default charset
+    msg = Message(sender="from@example.com",
+                  subject="subject",
+                  recipients=["foo@bar.com"])
+    msg.body = "normal ascii text"
+    assert 'Subject: =?' not in msg.as_string()
+    assert 'Content-Type: text/plain; charset="utf-8"' in msg.as_string()
+
+
+def test_empty_subject_header():
+    mail = Mail(settings)
+    msg = Message(sender="from@example.com",
+                  recipients=["foo@bar.com"])
+    msg.body = "normal ascii text"
+    mail.send(msg)
+    assert 'Subject:' not in msg.as_string()