I am not really sure.
what I have done.
1. change the email in the test domain to backup domain (which email is working), this 2 domains are in the server.
2. test sending email again in test server but it did not work. this is to prove that no issue on the php email function
3. pointing the database on test domain to back up domain to verify the issue. still had the issue
4. copy the source code from backup domain to the test domain, and pointing the database to back up domain then it still did not work,.
5. pointing back to test database and it works.
6. every step I also make change and save in setting.
I guess the issue would be some parameter did not save properly.
5. every step I make the setting in Mail as well