El Captain Mail Problems - Can't Receive Msg & Keeps Loading

Hi there, would like to see if somebody can help:

Using El Captain for a while but just all of sudden yesterday my Apple Mail had 2 issues:

1) Cannot receive new email (status bar always shows "Checking for mail"). Unless I close & open Mail again I start to receive new mails.
2) Many of my emails shown "loading". And it takes forever! I can only read very few messages without problem. The issue doesn't relate to attachment size as it happens to very small text only email as well.

Connection Doctor shows nothing wrong. I tried to reboot or even reinstall El Captain but can't solve the issues. All imap & smtp settings have never changed. It just happens all of sudden. Anybody can help or encountered the same problems?

Many thx for the assistance