email not working after yosemite install

Thank you, I have been through all that.

The problem is it keeps asking for the password. I KNOW I am using the correct email address and password. I can confirm this by checking my email with Mail2Web and also the fact that the account details work in iOS 8 on my iPhone and iPad, both devices can send and receive from the email that is failing on the MacBook.

I accidentally clicked on helpful, the answer from Linc has not helped my problem

I never resolved this problem despite being talked through by Apple for hours.

Been trying lots of times to add the missing Mail Account through System Preferences, still not able to get it.

I recently got a newer MacBookPro and had exactly the same problem, one email account missing,( I think it's a POP Account where the 2 that were okay were IMAP accounts).

Upgraded to El Capitan the same evening I bought the Laptop), same problem - only 2 Mail Accounts there. Kept trying, no joy.

Tried a different approach - opened the Mail app, went into the 'Mail' menu then clicked on 'Adding Account' - finally the account was there!!!!!!!!

I don't have the Mailboxes I set up originally, but at least I have made some progress and thought I should pass it on.

Basically if you have lost a Mail Account and are using System Preferences to get it back, give that up and try Adding the Account while in the Mail app.