mail server access problem

Thanks for the response. I checked out the link you sent and see that my settings already included automatic connection management. Re-entering my AT&T password had no improvement. Unless the other settings are incorrect I do not see where I can make further changes. Should the host name just be mail.att.net? I am running High Sierra. Here is what my settings screen looks like (different from the example for Sierra OS in the above link).

I have not intentionally changed any mail settings, although this seems to have started after upgrading to High Sierra from El Capitan. As far as I recall my sbcglobal.net address has always applied to this account, and my ISP has been AT&T for years. The only setting I see with my user name is in the box titled user name; if that should be changed what should it say?

One apparent difference from El Capitan seems to include more iCloud involvement than I noticed before; in my iCloud settings I have not checked the Mail app box since mail had been working fine. Could that have any impact on this issue?

My problem persists with my Apple Mail account accessing the IMAP server for both Mail and iCloud. Connection Doctor seems to indicate a connection ok with the SMTP server, but outgoing as well as incoming mail is currently unavailable. Logging in directly to my Yahoo internet mail account is working fine. Unless there is some combination of settings I can find in need of change I am thinking it may require my deleting and re-establishing my Apple Mail account. If this is needed, I could use some advice for doing that. Your thoughts?

Just tried that with no success. Unchecking the ENABLE THIS ACCOUNT and rechecking it cycled the red OFFLINE dot to clear for several seconds and then back to red.

P.S. I also tried using the plus symbol, but that just tried to add a new account and replied that this account already exists.