Cannot set up new email account in Mail

Open Internet Accounts in System Preferences and make sure the account doesn't exist there. If it does, delete it and then add the new account. If it doesn't work by selecting the provider (like Yahoo), then choose Other Account and try that.

Rejection usually means that something is wrong in the information. Be careful to make sure each letter is exact as well as symbols and periods. Also do not add any extra spaces as that will also cause a failure.

You might also check with your provider to make sure they are compatible with El Capitan.

You can try opening you KeyChain app in the Utilities folder and look for passwords for that account. Delete all you find and try again to set up the account.

If the account setup fails because of wrong information in one or more fields, another setup window should open with a more advanced setup. Try going through the info there and make sure all information such as server information is correct.

You can try creating a new user on the computer for testing purposes. After creating a test user, log into it and try creating the email account there. That will tell you if there is a problem with your user account.

Since you can set up other accounts with no problem and it does the same in the test user account, then the problem is either Suddenlink, wrong information, or non compatiblility with El Capitan. It doesn't appear to be a problem with the Mail application or KeyChain. Since it fails at the first screen which tells you that it doesn't recognize your user name, password or name as being correct, that would tend to mean that something isn't typed right or Suddenlink has a problem.

You also say it worked one day, and failed the next. That usually indicates a problem with the provider in many cases.

If you have access to another Mac, try adding the account there. You might also try with a hard wire connection to the internet instead of wireless.

Other things you might consider is to go to the webmail site and go to account settings. Look for security features that may be blocking access to that device.

I have a feeling that even if you if you reverted back to the previous Mac OS, you would still have problems. But if you have a TM backup, you can restore it from a previous date. Other than that, you would have to reinstall a previous OS from scratch. If you are referring to an earlier version of El Capitan, you would have to have a copy of the installer before the latest update.

Sorry I just can't think of anything else to tell you. I believe what you say about typing everything correctly. But since you can add other accounts with no problem and you said it worked one day, I can only assume it's something with Suddenlink. All providers will work fine at the webmail site, but it appears that some providers just have problems with Apple or some mail applications.

Post back after speaking with them.