diff --git a/core/mail/protocol.py b/core/mail/protocol.py
index 2259608..0bddfd0 100644
--- a/core/mail/protocol.py
+++ b/core/mail/protocol.py
@@ -205,6 +205,10 @@ def receive_email(envelope, log=None):
     #    log.warning("Header to does not match envelope to")
     #    log.info(f"Header to: {header_to}, envelope to: {envelope.rcpt_tos[0]}")
 
+    if Email.objects.filter(reference=header_message_id).exists():  # break before issue thread is created
+        log.warning("Email already exists")
+        raise Exception("Email already exists")
+
     recipient = envelope.rcpt_tos[0].lower() if envelope.rcpt_tos else header_to.lower()
     sender = envelope.mail_from if envelope.mail_from else header_from
     subject = parsed.get('Subject')
@@ -263,7 +267,7 @@ class LMTPHandler:
         content = None
         try:
             content = envelope.content
-            email, new, reply, thread = await database_sync_to_async(receive_email)(envelope, log)
+            email, new, reply, thread = await receive_email(envelope, log)
             log.info(f"Created email {email.id}")
             systemevent = await database_sync_to_async(SystemEvent.objects.create)(type='email received',
                                                                                    reference=email.id)