Chosen Solution
Macbook Air “upgraded” to “High Sierra” OS and somehow lost one of my two email addresses (same account). Attempts to reinstate it via “Internet Accounts” system preferences only results in message “unable to verify account name or password.” All suggestions on “fix” gratefully received!
Turns out the answer to my problem is: If rebuilding the mailbox should fail, quit Mail.app and then delete “/Users/yourusername/Library/Mail/Envelope Index.” When you relaunch Mail.app, it will reimport all of your mail from scratch, effectively rebuilding every index for every mailbox.
Are you sure the mail box was still active? Using your phone or tablet are you able to access the mailbox? If you are you should be able to add it to your MacBook Air settings.
