"imap.mail.yahoo.com" problem

The most expedient way to fix that problem is to remove the problematic Mail Account, and then add it again.

First remove: Remove email accounts in Mail on Mac - Apple Support

Then add: Add email accounts in Mail on Mac - Apple Support

For Mail troubleshooting in general, read If you can’t send or receive email on your Mac - Apple Support

Ok, but we are supposed to pay attention to Apple's Important notes.

Important: If you’re unsure whether messages you want to keep are still on the mail server, move or copy them to a mailbox stored on your Mac (the mailbox appears in the On My Mac section in the Mail sidebar) before you remove the account in Mail.

Is Mail sending and receiving Yahoo! Mail messages now?

yahoo use a one time passcode to enable your account:

1. log into your yahoo account using Safari browser
2. Click Generate app password or Generate and manage app passwords.
3. Enter Apple Mail as the client application in the text field.
4. Click Generate password.
5. Follow the instructions below the app password.
6. Click Done.

Use this app password and your email address to sign in to Apple Mail app

will213A wrote:

"imap.mail.yahoo.com" not responding. "Cannot connect to IMAP server" Not sending or receiving yahoo emails. What is correct information in account preferences. Was fine 3 days ago!

Contact yahoo or look at their support site for the correct settings.