It just changed back from a period of time suddenly. Initially I think I did not save it but looks like it's not the case as after I reload the module, it shown the changes I made, but for a period of time, it just revert back the change I made suddenly. As I do not make changes on the configure of the module very often (just sometimes add or remove mailbox or reset of password), so the configure I made is obvious if it gets revert back.