diff --git a/chatmaild/src/chatmaild/doveauth.py b/chatmaild/src/chatmaild/doveauth.py
index 895ca6d2..314e7348 100644
--- a/chatmaild/src/chatmaild/doveauth.py
+++ b/chatmaild/src/chatmaild/doveauth.py
@@ -1,8 +1,11 @@
 import json
 import logging
 import os
+import re
 import sys
 
+import filelock
+
 try:
     import crypt_r
 except ImportError:
@@ -13,6 +16,7 @@ from .dictproxy import DictProxy
 from .migrate_db import migrate_from_db_to_maildir
 
 NOCREATE_FILE = "/etc/chatmail-nocreate"
+VALID_LOCALPART_RE = re.compile(r"^[a-z0-9._-]+$")
 
 
 def encrypt_password(password: str):
@@ -52,6 +56,10 @@ def is_allowed_to_create(config: Config, user, cleartext_password) -> bool:
         )
         return False
 
+    if not VALID_LOCALPART_RE.match(localpart):
+        logging.warning("localpart %r contains invalid characters", localpart)
+        return False
+
     return True
 
 
@@ -140,8 +148,13 @@ class AuthDictProxy(DictProxy):
         if not is_allowed_to_create(self.config, addr, cleartext_password):
             return
 
-        user.set_password(encrypt_password(cleartext_password))
-        print(f"Created address: {addr}", file=sys.stderr)
+        lock = filelock.FileLock(str(user.password_path) + ".lock", timeout=5)
+        with lock:
+            userdata = user.get_userdb_dict()
+            if userdata:
+                return userdata
+            user.set_password(encrypt_password(cleartext_password))
+            print(f"Created address: {addr}", file=sys.stderr)
         return user.get_userdb_dict()