I upgraded my iMac to macOS High Sierra 10.13.4. Since then my email account with 1&1 (that was previously working fine) - decided to work intermittently, then just failed completely. I contacted 1&1 and they couldn't help, saying I was using the right login credentials. They even screen shared and were still none the wiser and said it must be an internal problem.
I deleted the account from Mail.app and tried to Add Account - but it just says Unable to verify account or password. If I ignore that and just carry on filling in the appropriate fields, I get another window Select the Apps you want to use with this account. I check Mail and press next and there's spinning beachball connection icon for a split second and then nothing happens.
How do I get my mail account with 1&1 to work again with Mail.app?