Apple Mail.app have problem when connecting to iCloud mail server

iMac (Retina 5K, 27-inch, Late 2015),   System Version: macOS 10.15.4 (19E287)

From yesterday, it keeps giving the error message "Connections to host p28-imap.mail.icloud.com.cn on the default ports failed." when I try to get email from my iCloud mailbox.

I configured the mail server automatically with iCloud preferences, and it worked for months until yesterday. I have reboot my Mac, uncheck and re-check the mail option in iCloud preference, but no help.

The only similar case I could find is this one: https://discussions.apple.com/thread/8433058

It suggest to check on the mail server setting, but the only setting I can modify is the SMTP server setting in Mail Preferences.

Any idea for other possible reason or solution?