MAC CLIENT 8.2 (older Apple) not receiving incoming emails (IMAP)

Today, our MAIL CLIENT stopped downloading the emails after a website server maintenance downtime. We can send but we cannot receive them using Mail Client. We checked with the server customer service, and all settings are okay. We don't know what's causing the issue. We keep getting a pop-up window from our Internet Account Preferences saying it can't connect to our email account and asking us to enter the PW again. We do, but it cannot verify the account and email. Even though that's the PW is the right one. But we cannot understand what PW they require. When we open the? it says something about the Apple Certificate. Can you help us debug this? Any help or suggestions are highly appreciated. Thanks.