diff --git a/modoboa/admin/models/mailbox.py b/modoboa/admin/models/mailbox.py
index fd2612ab1..f4159933f 100644
--- a/modoboa/admin/models/mailbox.py
+++ b/modoboa/admin/models/mailbox.py
@@ -322,7 +322,8 @@ class Mailbox(mixins.MessageLimitMixin, AdminObject):
                 not user.userobjectlimit_set.get(name="quota").max_value
             )
             self.set_quota(values["quota"], override_rules)
-        self.message_limit = values.get("message_limit")
+        if values.get("message_limit", "absent") != "absent":
+            self.message_limit = values["message_limit"]
         if newaddress:
             self.rename(local_part, domain)
         self.save()