IMAP settings Mac

First, while it may sound too simple, have you tried restarting the computer?

Second, assuming this isn't an iCloud account, please try going to System Preferences > Internet Accounts and removing and reinstating the account in question.

If that doesn't help, or you get stuck reinstating the account, it would help if you could give more detail:

1. Are you seeing a particular error?
2. Who is your email provider? For example, AOL, Gmail, Hotmail (Outlook) or another.
3. Are you able to access your mail by visiting the provider's website in a browser like Safari or Chrome?

Finally, it would be good to ensure you're running the latest version of macOS. To check, click the Apple menu – top-left of the screen – and choose About This Mac. At the time of writing it should be macOS Catalina 10.15.4.

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.

You can save us asking dozens of questions by submitting a report...

EtreCheck is a simple little app to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac.

http://www.etresoft.com/etrecheck

Pastebin is a good place to paste the whole report if you capture the URL while there…

https://pastebin.com/

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck.

There is also free Malwarebytes which may take care of it...

https://www.malwarebytes.com/mac/

So, what about the answers to my three questions and your macOS version?