diff --git a/backend/src/services/emailProvider/emailProviderService.ts b/backend/src/services/emailProvider/emailProviderService.ts
index 3381801a..0ed2c4be 100644
--- a/backend/src/services/emailProvider/emailProviderService.ts
+++ b/backend/src/services/emailProvider/emailProviderService.ts
@@ -80,7 +80,24 @@ export interface CreateProviderConfigData {
   isDefault?: boolean;
 }
 
+// Validiert Domain-Format (z.B. stressfrei-wechseln.de, mail.beispiel.com)
+const DOMAIN_REGEX = /^[a-z0-9]([a-z0-9-]*[a-z0-9])?(\.[a-z0-9]([a-z0-9-]*[a-z0-9])?)+$/;
+
+function validateDomain(domain: string | undefined): string {
+  if (!domain || !domain.trim()) {
+    throw new Error('Domain ist erforderlich');
+  }
+  const normalized = domain.trim().toLowerCase();
+  if (!DOMAIN_REGEX.test(normalized)) {
+    throw new Error(`Ungültige Domain: "${domain}". Format: name.tld (z.B. meine-firma.de)`);
+  }
+  return normalized;
+}
+
 export async function createProviderConfig(data: CreateProviderConfigData) {
+  // Domain validieren
+  const validatedDomain = validateDomain(data.domain);
+
   // Falls isDefault=true, alle anderen auf false setzen
   if (data.isDefault) {
     await prisma.emailProviderConfig.updateMany({
@@ -102,7 +119,7 @@ export async function createProviderConfig(data: CreateProviderConfigData) {
       apiKey: data.apiKey || null,
       username: data.username || null,
       passwordEncrypted,
-      domain: data.domain,
+      domain: validatedDomain,
       defaultForwardEmail: data.defaultForwardEmail || null,
       imapEncryption: data.imapEncryption ?? 'SSL',
       smtpEncryption: data.smtpEncryption ?? 'SSL',
@@ -135,7 +152,7 @@ export async function updateProviderConfig(
   if (data.apiUrl !== undefined) updateData.apiUrl = data.apiUrl;
   if (data.apiKey !== undefined) updateData.apiKey = data.apiKey || null;
   if (data.username !== undefined) updateData.username = data.username || null;
-  if (data.domain !== undefined) updateData.domain = data.domain;
+  if (data.domain !== undefined) updateData.domain = validateDomain(data.domain);
   if (data.defaultForwardEmail !== undefined)
     updateData.defaultForwardEmail = data.defaultForwardEmail || null;
   if (data.imapEncryption !== undefined) updateData.imapEncryption = data.imapEncryption;
diff --git a/frontend/src/pages/settings/EmailProviders.tsx b/frontend/src/pages/settings/EmailProviders.tsx
index bec12819..04518e26 100644
--- a/frontend/src/pages/settings/EmailProviders.tsx
+++ b/frontend/src/pages/settings/EmailProviders.tsx
@@ -101,6 +101,7 @@ export default function EmailProviders() {
       emailProviderApi.createConfig(data),
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['email-provider-configs'] });
+      queryClient.invalidateQueries({ queryKey: ['email-provider-public-settings'] });
       closeModal();
     },
   });
@@ -110,6 +111,7 @@ export default function EmailProviders() {
       emailProviderApi.updateConfig(id, data),
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['email-provider-configs'] });
+      queryClient.invalidateQueries({ queryKey: ['email-provider-public-settings'] });
       closeModal();
     },
   });
@@ -118,6 +120,7 @@ export default function EmailProviders() {
     mutationFn: (id: number) => emailProviderApi.deleteConfig(id),
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['email-provider-configs'] });
+      queryClient.invalidateQueries({ queryKey: ['email-provider-public-settings'] });
     },
   });
 
@@ -547,13 +550,27 @@ export default function EmailProviders() {
                   </div>
                 </div>
 
-                <Input
-                  label="Domain *"
-                  value={formData.domain}
-                  onChange={(e) => setFormData({ ...formData, domain: e.target.value })}
-                  placeholder="stressfrei-wechseln.de"
-                  required
-                />
+                <div>
+                  <Input
+                    label="Domain *"
+                    value={formData.domain}
+                    onChange={(e) =>
+                      setFormData({ ...formData, domain: e.target.value.toLowerCase().trim() })
+                    }
+                    placeholder="stressfrei-wechseln.de"
+                    required
+                    pattern="^[a-z0-9]([a-z0-9\-]*[a-z0-9])?(\.[a-z0-9]([a-z0-9\-]*[a-z0-9])?)+$"
+                    title="Gültige Domain erforderlich, z.B. meine-firma.de oder mail.beispiel.com"
+                  />
+                  {formData.domain &&
+                    !/^[a-z0-9]([a-z0-9\-]*[a-z0-9])?(\.[a-z0-9]([a-z0-9\-]*[a-z0-9])?)+$/.test(
+                      formData.domain,
+                    ) && (
+                      <p className="text-xs text-red-600 mt-1">
+                        Keine gültige Domain – Format: name.tld (z.B. meine-firma.de)
+                      </p>
+                    )}
+                </div>
 
                 <Input
                   label="Bezeichnung für Kunden-E-Mails (UI-Label)"