diff --git a/app/flask-postgres/app/app.py b/app/flask-postgres/app/app.py
index 3f39c8f..215c4f2 100644
--- a/app/flask-postgres/app/app.py
+++ b/app/flask-postgres/app/app.py
@@ -1,5 +1,6 @@
 import logging
 import os
+import re
 from datetime import datetime
 from logging.handlers import RotatingFileHandler
 
@@ -414,10 +415,49 @@ def admin_mandanten():
             kontakt_email = request.form.get("kontakt_email", "").strip()
             level = request.form.get("level", "0").strip()
 
+            email_pattern = r"^[^@\s]+@[^@\s]+\.[^@\s]+$"
+
+            error_message = None
+
+            if not kuerzel:
+                error_message = "Kürzel ist ein Pflichtfeld."
+            elif not name:
+                error_message = "Name ist ein Pflichtfeld."
+            elif not kontakt_email:
+                error_message = "Kontakt E-Mail ist ein Pflichtfeld."
+            elif not re.match(email_pattern, kontakt_email):
+                error_message = "Bitte eine gültige Kontakt-E-Mail eingeben."
+
+            if error_message:
+                cur.execute("""
+                    SELECT id, kuerzel, name, kontakt_email, level
+                    FROM mandant
+                    ORDER BY id
+                """)
+                mandanten = fetchall_dict(cur)
+
+                cur.close()
+                conn.close()
+
+                return render_template(
+                    "admin_mandanten.html",
+                    page_title="Admin - Mandanten",
+                    active_page="admin",
+                    mandanten=mandanten,
+                    form_error=error_message,
+                    form_values={
+                        "kuerzel": kuerzel,
+                        "name": name,
+                        "kontakt_email": kontakt_email,
+                        "level": level,
+                    },
+                    **get_current_user()
+                )
+
             cur.execute("""
                 INSERT INTO mandant (kuerzel, name, kontakt_email, level)
                 VALUES (%s, %s, %s, %s)
-            """, (kuerzel, name, kontakt_email or None, int(level or 0)))
+            """, (kuerzel, name, kontakt_email, int(level or 0)))
             conn.commit()
 
         elif action == "update":
diff --git a/app/flask-postgres/app/templates/admin_mandanten.html b/app/flask-postgres/app/templates/admin_mandanten.html
index d29f29f..34c1fe2 100644
--- a/app/flask-postgres/app/templates/admin_mandanten.html
+++ b/app/flask-postgres/app/templates/admin_mandanten.html
@@ -14,26 +14,49 @@
     </div>
 
     <form method="post" class="admin-grid-form" id="mandant-create-form" novalidate>
+
+        {% if form_error %}
+            <div class="form-row-full">
+                <div class="error-box">{{ form_error }}</div>
+            </div>
+        {% endif %}
+
   <input type="hidden" name="action" value="create">
 
   <div class="form-row">
     <label for="kuerzel">Kürzel</label>
-    <input type="text" id="kuerzel" name="kuerzel" required>
-  </div>
+    <input type="text"
+        id="kuerzel"
+        name="kuerzel"
+        value="{{ form_values.kuerzel if form_values else '' }}"
+        required>
+    </div>
 
   <div class="form-row">
     <label for="name">Name</label>
-    <input type="text" id="name" name="name" required>
+    <input type="text"
+        id="name"
+        name="name"
+        value="{{ form_values.name if form_values else '' }}"
+        required>
   </div>
 
   <div class="form-row">
     <label for="kontakt_email">Kontakt E-Mail</label>
-    <input type="email" id="kontakt_email" name="kontakt_email" required>
+    <input type="email"
+        id="kontakt_email"
+        name="kontakt_email"
+        value="{{ form_values.kontakt_email if form_values else '' }}"
+        required>
   </div>
 
   <div class="form-row">
     <label for="level">Level</label>
-    <input type="number" id="level" name="level" value="0" min="0">
+    <input type="number"
+       id="level"
+       name="level"
+       value="{{ form_values.level if form_values else '0' }}"
+       min="0">
   </div>
 
   <div class="form-row form-row-full">
@@ -152,4 +175,4 @@
       }
     });
   });
-</script>
\ No newline at end of file
+</script>