diff --git a/cspell.json b/cspell.json
index b84dba2..399025e 100644
--- a/cspell.json
+++ b/cspell.json
@@ -22,8 +22,9 @@
         "Reparateure",
         "reparateurs",
         "Reparateurs",
-        "tabindex"
+        "tabindex",
+        "johnpassword"
     ],
     "ignoreWords": [],
     "import": []
-}
+}
\ No newline at end of file
diff --git a/repapp/email_interface/apps.py b/repapp/email_interface/apps.py
index 1548cf3..24caa3e 100644
--- a/repapp/email_interface/apps.py
+++ b/repapp/email_interface/apps.py
@@ -6,12 +6,14 @@ from django.apps import AppConfig
 logger = logging.getLogger(__name__)
 
 
-def message_answered_receiver(sender, instance, **kwargs):
+def message_answered_receiver(sender, instance, **kwargs):  # pragma: no cover
+    # used for manual testing only
     logger.debug('Message answered signal received: %s %s',
                  instance, kwargs['answer'])
 
 
-def new_message_receiver(sender, instance, **kwargs):
+def new_message_receiver(sender, instance, **kwargs):  # pragma: no cover
+    # used for manual testing only
     logger.debug('New message signal received: %s', instance)
 
 
diff --git a/repapp/email_interface/forms.py b/repapp/email_interface/forms.py
index 069e096..7fde9bf 100644
--- a/repapp/email_interface/forms.py
+++ b/repapp/email_interface/forms.py
@@ -23,17 +23,23 @@ class MessageForm(forms.ModelForm):
         fields = ['receiver', 'summary', 'html_content']
         widgets = {'html_content': CKEditorUploadingWidget()}
 
-    def clean(self):
-        if self.cleaned_data['use_mail_text'] and self.cleaned_data['mail_text'] == '':
+    def clean_mail_text(self):
+        text = self.cleaned_data["mail_text"]
+        use_text = self.data["use_mail_text"]
+
+        logger.debug(
+            'MessageForm clean_mail_text: use text: %r, text: %s', use_text, text)
+
+        if use_text and not text:
             raise forms.ValidationError(
                 _("Mail text cannot be empty if it is used!"))
 
-        return super().clean()
+        return text
 
     def save(self, commit: bool = True):
         message = super().save(commit=False)
 
-        if not self.cleaned_data['use_mail_text']:
+        if self.cleaned_data['use_mail_text']:
             message.text_content = self.cleaned_data['mail_text']
         else:
             # convert HTML content to text content
diff --git a/repapp/email_interface/tests.py b/repapp/email_interface/tests.py
index 7ce503c..a9644c8 100644
--- a/repapp/email_interface/tests.py
+++ b/repapp/email_interface/tests.py
@@ -1,3 +1,91 @@
+import logging
 from django.test import TestCase
+from django.contrib.auth import get_user_model
+from django import forms
+from .forms import MessageForm
 
-# Create your tests here.
+
+logger = logging.getLogger(__name__)
+
+
+class MessageFormTestCase(TestCase):
+    """
+    Tests all logic contained in the OneTimeLogin model.
+    """
+
+    def setUp(self):
+        user = get_user_model().objects.create_user(
+            "john",
+            "lennon@thebeatles.com",
+            "johnpassword")
+        user.save()
+
+    def test_message_from_ok_no_text(self):
+        user = get_user_model().objects.get(username='john')
+
+        form = MessageForm({
+            'receiver': user.pk,
+            'summary': 'My mail summary',
+            'html_content': '<p>Hallo, Welt!</p>',
+            'mail_text': '',
+            'use_mail_text': False,
+        })
+
+        assert form.is_valid(), 'form is valid'
+
+        message = form.save(commit=False)
+
+        logger.debug('Message: %s', message)
+        logger.debug('Message text: %s', message.text_content)
+
+        assert message.pk is None, 'message was not saved'
+        assert message.receiver == user, 'receiver is set'
+        assert message.sender is None, 'no sender is set'
+        assert message.summary == 'My mail summary', 'summary is set'
+        assert message.html_content == '<p>Hallo, Welt!</p>', 'HTML is set'
+        assert 'Hallo, Welt!' in message.text_content, 'text was generated'
+
+        message = form.save()
+
+        assert message.pk is not None, 'message was saved'
+
+    def test_message_from_ok_text(self):
+        user = get_user_model().objects.get(username='john')
+
+        form = MessageForm({
+            'receiver': user.pk,
+            'summary': 'My mail summary',
+            'html_content': '<p>Hallo, Welt!</p>',
+            'mail_text': 'Hallo!',
+            'use_mail_text': True,
+        })
+
+        assert form.is_valid(), 'form is valid'
+
+        message = form.save(commit=False)
+
+        logger.debug('Message: %s', message)
+        logger.debug('Message text: %s', message.text_content)
+
+        assert message.pk is None, 'message was not saved'
+        assert message.receiver == user, 'receiver is set'
+        assert message.sender is None, 'no sender is set'
+        assert message.summary == 'My mail summary', 'summary is set'
+        assert message.html_content == '<p>Hallo, Welt!</p>', 'HTML is set'
+        assert message.text_content == 'Hallo!', 'text was used'
+
+    def test_message_from_wrong_no_text(self):
+        user = get_user_model().objects.get(username='john')
+
+        form = MessageForm({
+            'receiver': user.pk,
+            'summary': 'My mail summary',
+            'html_content': '<p>Hallo, Welt!</p>',
+            'mail_text': '',
+            'use_mail_text': True,
+        })
+
+        logger.debug('Form data: text: %r', form)
+
+        assert not form.is_valid()
+        assert form.has_error('mail_text')