diff --git a/email_api/email_api/app.py b/email_api/email_api/app.py
index dc205f6..fbc0aee 100644
--- a/email_api/email_api/app.py
+++ b/email_api/email_api/app.py
@@ -53,6 +53,17 @@ def domain_exists(domain):
     # domains ist eine Liste von Objekten mit key 'domain_name'
     return any(d.get('domain_name') == domain for d in domains)
 
+def inbox_exists(domain, local_part):
+    """
+    Prüft, ob für `local_part@domain` ein Postfach angelegt ist.
+    """
+    url = f"{MAILCOW_API}/get/mailbox/all/{domain}"
+    headers = {'X-API-Key': API_KEY}
+    resp = requests.get(url, headers=headers, timeout=5)
+    resp.raise_for_status()
+    mailboxes = resp.json()
+    return any(m.get('local_part') == local_part for m in mailboxes)
+
 def mark_email_as_processed(bucket, key, status, processor='rest-api'):
     """Setzt processed-Metadaten auf einen beliebigen Status."""
     try:
@@ -144,7 +155,7 @@ def process_email(domain):
     data = request.get_json()
     if not data:
         return jsonify({'error': 'Invalid payload'}), 400
-    
+
     request_id = data.get('request_id', 'no-request-id')
     
     payload_summary = {
@@ -157,35 +168,49 @@ def process_email(domain):
         f"[{request_id}] INCOMING POST /process/{domain}: "
         f"payload_summary={payload_summary}"
     )
-    
+
+    # 1) E-Mail decodieren und parsen wie gehabt
     content = data.get('email_content')
     compressed = data.get('compressed', False)
     raw = base64.b64decode(content)
     email_bytes = gzip.decompress(raw) if compressed else raw
-    
+
     msg = BytesParser(policy=default).parsebytes(email_bytes)
     from_addr = getaddresses(msg.get_all('from', []))[0][1] if msg.get_all('from') else f'lambda@{domain}'
-
     recipients = []
-    for hdr in ('to', 'cc', 'bcc'):
+    for hdr in ('to','cc','bcc'):
         recipients += [addr for _n, addr in getaddresses(msg.get_all(hdr, []))]
 
-    logger.debug(f"[{request_id}] Parsed email: from={from_addr}, recipients={recipients}")    
-
     if not recipients:
         return jsonify({'error': 'No recipients'}), 400
 
-    # Domain-Check
-    recipient_domains = {addr.split('@')[-1] for addr in recipients}
-    if not any(domain_exists(d) for d in recipient_domains):
-        logger.info(f"[{request_id}] No known recipient domains ({recipient_domains}) – skip.")
-        return jsonify({'message': 'Unknown domain – skipped'}), 404
+    # 2) Filter: nur Postfächer der angefragten Domain, die auch existieren
+    valid_recipients = []
+    for addr in recipients:
+        try:
+            local, dom = addr.split('@', 1)
+        except ValueError:
+            continue
+        if dom.lower() != domain.lower():
+            # andere Domain: überspringen
+            continue
+        if inbox_exists(domain, local):
+            valid_recipients.append(addr)
+        else:
+            logger.info(f"Skipping non-existent inbox: {addr}")
 
+    if not valid_recipients:
+        logger.info(f"[{request_id}] Keine gültigen Inboxes für {domain} – skip.")
+        return jsonify({'message': 'No valid inboxes – skipped'}), 404
+
+    # 3) Senden an die gefilterten Adressen
     with smtplib.SMTP(SMTP_HOST, SMTP_PORT) as smtp:
-        smtp.sendmail(from_addr, recipients, email_bytes)
+        smtp.sendmail(from_addr, valid_recipients, email_bytes)
 
-    # Keine Markierung hier mehr; übernimmt Lambda
-    return jsonify({'message': 'Email forwarded', 'recipients': recipients}), 200
+    return jsonify({
+        'message': 'Email forwarded',
+        'forwarded_to': valid_recipients
+    }), 200
 
 
 @app.route('/retry/<domain>', methods=['GET'])