diff --git a/email_api/email_api/app.py b/email_api/email_api/app.py
index 1839088..8c94ff4 100644
--- a/email_api/email_api/app.py
+++ b/email_api/email_api/app.py
@@ -226,9 +226,15 @@ def retry_domain_emails(domain):
     if auth != f'Bearer {API_TOKEN}':
         return jsonify({'error': 'Unauthorized'}), 401
 
+    # 1) Domain-Check ganz am Anfang
+    if not domain_exists(domain):
+        logger.info(f"Retry aborted: unknown domain '{domain}'")
+        return jsonify({'error': f"Unknown domain '{domain}'"}), 404
+
     bucket = domain.replace('.', '-') + '-emails'
     paginator = s3_client.get_paginator('list_objects_v2')
 
+    # 2) alle unprocessed Keys sammeln
     unprocessed = []
     for page in paginator.paginate(Bucket=bucket):
         for obj in page.get('Contents', []):
@@ -245,24 +251,30 @@ def retry_domain_emails(domain):
         try:
             body = s3_client.get_object(Bucket=bucket, Key=key)['Body'].read()
             msg = BytesParser(policy=default).parsebytes(body)
-
             from_addr = (
                 getaddresses(msg.get_all('from', []))[0][1]
                 if msg.get_all('from') else f'retry@{domain}'
             )
-            to_addrs  = [a for _n, a in getaddresses(msg.get_all('to', []))]
-            cc_addrs  = [a for _n, a in getaddresses(msg.get_all('cc', []))]
-            bcc_addrs = [a for _n, a in getaddresses(msg.get_all('bcc', []))]
-            recipients = to_addrs + cc_addrs + bcc_addrs
+
+            # Sammle alle To/Cc/Bcc
+            recipients = []
+            for hdr in ('to', 'cc', 'bcc'):
+                recipients += [addr for _n, addr in getaddresses(msg.get_all(hdr, []))]
 
             if not recipients:
-                results['failed'].append({'key': key, 'error': 'no recipients'})
+                # gar keine Adressen → überspringen
+                mark_email_as_processed(bucket, key, 'unknownDomain')
+                results['processed'].append(key)
+                results['failed'].append({
+                    'key': key,
+                    'status': 'unknownDomain',
+                    'reason': 'no recipients'
+                })
                 continue
 
-            # **Neu: Domain-Match prüfen**
-            domains = {addr.split('@')[-1].lower() for addr in recipients}
-            if domain.lower() not in domains:
-                # niemals zur eigenen Domain adressiert → unknownDomain
+            # 3) Domain-Match: nur Mails, die an die angefragte Domain adressiert sind
+            domains_in_mail = {addr.split('@')[-1].lower() for addr in recipients if '@' in addr}
+            if domain.lower() not in domains_in_mail:
                 mark_email_as_processed(bucket, key, 'unknownDomain')
                 results['processed'].append(key)
                 results['failed'].append({
@@ -273,48 +285,66 @@ def retry_domain_emails(domain):
                 })
                 continue
 
-            # mindestens ein passender Empfänger → Sendeversuch
+            # 4) Inbox-Check: nur existierende Postfächer zulassen
+            valid_recipients = []
+            for addr in recipients:
+                try:
+                    local, dom = addr.split('@', 1)
+                except ValueError:
+                    continue
+                if dom.lower() == domain.lower() and inbox_exists(domain, local):
+                    valid_recipients.append(addr)
+                else:
+                    logger.info(f"Skipping non-existent inbox: {addr}")
+
+            if not valid_recipients:
+                mark_email_as_processed(bucket, key, 'unknownUser')
+                results['processed'].append(key)
+                results['failed'].append({
+                    'key': key,
+                    'status': 'unknownUser',
+                    'from': from_addr,
+                    'to': recipients
+                })
+                continue
+
+            # 5) Versand an die validierten Adressen
             try:
                 with smtplib.SMTP(SMTP_HOST, SMTP_PORT) as smtp:
-                    smtp.sendmail(from_addr, recipients, body)
+                    smtp.sendmail(from_addr, valid_recipients, body)
                 mark_email_as_processed(bucket, key, 'true')
                 results['processed'].append(key)
 
             except smtplib.SMTPRecipientsRefused as e:
-                refused = e.recipients
-                status = (
-                    'unknownUser'
-                    if any(addr.split('@')[-1].lower() == domain.lower() for addr in refused)
-                    else 'unknownDomain'
-                )
-                # bytes → str
-                clean = {addr: {'code': code, 'message': (msg.decode('utf-8', 'ignore')
-                                                          if isinstance(msg, bytes) else str(msg))}
-                         for addr, (code, msg) in refused.items()}
-
-                mark_email_as_processed(bucket, key, status)
+                # falls Mailcow einzelne Adressen ablehnt
+                mark_email_as_processed(bucket, key, 'unknownUser')
+                refused = {
+                    addr: {'code': code, 'message': msg.decode('utf-8','ignore') if isinstance(msg, bytes) else str(msg)}
+                    for addr, (code, msg) in e.recipients.items()
+                }
                 results['processed'].append(key)
                 results['failed'].append({
                     'key': key,
-                    'status': status,
+                    'status': 'unknownUser',
                     'from': from_addr,
-                    'to': recipients,
-                    'refused': clean
+                    'to': valid_recipients,
+                    'refused': refused
                 })
 
             except Exception as e:
-                # andere SMTP-Fehler behandeln wie unknownDomain
+                # alle anderen SMTP-Fehler behandeln wir als unknownDomain
                 mark_email_as_processed(bucket, key, 'unknownDomain')
                 results['processed'].append(key)
                 results['failed'].append({
                     'key': key,
                     'status': 'unknownDomain',
                     'from': from_addr,
-                    'to': recipients,
+                    'to': valid_recipients,
                     'error': str(e)
                 })
 
         except Exception as e:
+            # Parsing- oder S3-Fehler
             results['failed'].append({'key': key, 'error': str(e)})
 
     return jsonify(results), 200