How to stop mail from automatically editing SMTP servers?

Hi, I've been having some trouble with the built-in Mail application on my MacBook Pro with Retina Display for the past a couple of months. Incoming mails work normally but I have trouble sending emails using my 163 account. It keeps giving me the following error message and asking me to select a server

"This message could not be sent because your account does not have a preferred outgoing mail server. Select an outgoing mail server from the list below."

I have tried to select the server in Mail/Preferences/Server Setting/Outgoing Mail Account. But it won't remember my choices and will reset to "None" after a couple of seconds:

The official site of site instructed that the outgoing server needs to be updated from the default "applesmtp.163.com" to "smtp.163.com". And I did uncheck "Automatically manage connection settings" and enter the actual account and password. However, no matter how many times I try, it either reverts to the default or creates a duplicate of the default... 😟

I think the problem is Mail keeps automatically setting the SMTP server, but I have no idea how to stop it. I have this problem for both my gmail account and my 163 account. The smpt server list window no longer has a box “Automatically detect and maintain account settings” for me to un-check. And if I try to set to for too many times, Mail will crash.

This is really driving me nuts because I have to open the webpage and log in whenever I need to send a reply. I truly appreciate any advice or help!