Thanks for the reply. I first tried my usual Yahoo password. Then, per the suggestion on the “Fixed Yahoo Account” page, I got an app password from Yahoo and tried it. Same problem.
What seems odd to me is that under Accounts, there is no username or password field in either the POP3 tab or SMTP tab. These fields do appear in the CardDAV and CalDAV tabs. I wonder if this is a bug in the program.
In Windows Live Mail, that I am trying to move from, the username and password fields appear on the same tab as the POP3 and SMTP servers.
If you are using IMAP, could you verify whether there is a password field in the Accounts>IMAP tab?