Mail won't access account

I had previously set up a Mail account with my RCN server (I've got a late 2012 iMac, recently upgraded OS to Catalina 10.15.6) and was receiving and sending e-mail fine. I got confused by another Mail account I was having trouble accessing and accidentally changed the password, to my RCN mail. When I couldn't get e-mail, I changed the RCN mail password back, but I couldn't get my iMac to receive or send e-mail on the RCN server, even though I can log into my RCN webmail via browser. So, I deleted the RCN mail account (it just had a few unimportant e-mails, still on the RCN webmail server) and tried to add a new RCN account. Mail won't do it! I've entered the right e-mail address and password and IMAP account type and such but I just get that spinning pizza wheel that never ends...

Note that I was able to set up my RCN mail on another iMac computer with OS 10.13 (High Sierra), so it's got to be an OS Catalina problem. I've tried turning off Mail, shutting down the computer and restarting, rebuilding the mailbox. Anyone else have this sort of problem.