diff --git a/contrib/testing/cyrus-fixture.py b/contrib/testing/cyrus-fixture.py
index d9194b8..e863dd5 100755
--- a/contrib/testing/cyrus-fixture.py
+++ b/contrib/testing/cyrus-fixture.py
@@ -222,7 +222,10 @@ def ensure_mailboxes(password: str) -> None:
     try:
         client.login("testadmin", password)
         for username in SOURCE_USERS:
-            mailbox = f"user.{username}"
+            # altnamespace uses '/' as the hierarchy delimiter.  A literal
+            # "user.name" would create a shared top-level mailbox and LMTP
+            # would correctly reject delivery to the intended user.
+            mailbox = f"user/{username}"
             status, _ = client.create(mailbox)
             if status not in ("OK", "NO"):
                 raise RuntimeError(f"could not create Cyrus mailbox {mailbox}")