iCloud suddenly not connecting to SMTP or IMAP

I've been using Mail for years with a yahoo account being my main email address. I have an iCloud email but do not use it. Suddenly I'm getting a notice that "This Mac is not connecting to iCloud because of a problem with [email]. Go to iCloud setting to update password." I do this and put in my password and it seems that all is fine. When I go to Mail, I can't send out emails without getting the same message. I have tried the connection doctor and it shows that I can connect to the yahoo (now AT&T) server incoming and outgoing but connection to iCloud SMTP and IMAP have failed.

I have tried so many ways to go around this. 1)Making sure my iOS is at the latest (10.11.6); 2) moving .plist files and cache folders from the library to my desktop and rebooting; 3) making sure the settings are all correct for my yahoo pop account as well as the iCloud IMAP account. I'm not real sure about logging out of iCloud because of the warning of deleting files from my computer. I really can't afford for anything to get lost even though I do have a backup.

I am able to access my iCloud account online as well as all my emails through Yahoo's website. I can access everything on my iPhone as well.

Please advise on how to rectify this! I've lost a lot of time trying to work through this and am completely at a loss on how to resolve this issue. Would updating to Sierra be a goo move to try? Maybe that will force iCloud to update itself?

Thanks!