After 2+ hours on the phone with senior Apple tech, here’s a work-around that worked for me:
1. Open Keychain and search for gmail
2. Delete any entries related to your failed email address
3. Search for Google
4. Delete any entries related to your failed email account
5. Close Keychain and Apple mail if it is open and restart your machine
6. Go to System Preferences, Internet Accounts and delete your failed email account (-)
7. Add a new account (+) only do NOT select Google, choose Add Other Account, then @ Mail Account
8. Enter your name, email address of your Gmail account, and the password for your Gmail account
9. For incoming Hostname enter map.gmail.com, for Outgoing Server enter smtp.gmail.com
10. That should generate a proper account in your accounts list
11. Close preferences, rsetart again and open Mail
Hopefully that solves your problem, it did mine.
Longer term will require a fix by Apple
On Mar 27, 2019, at 2:13 PM, Apple Support Communities <discussions-updates@email.apple.com> wrote:
Apple Support CommunitiesScott Willsey has posted in the macOS Mojave community.Mojave 10.14.4 fails to Authenticate Gmail accountIt's perfectly valid to be frustrated about this issue, but this has NOTHING TO DO with whether or not Apple is making tv shows. Those two Sony guys? They don't know how to write email clients or handle oauth or do anything related to email other than fire one off to Reese Witherspoon and Big Bird to see if they're available for a project or two.
Apple has always had bugs in their software, like everyone else, and they always will. My first personally owned computer was a 128k Mac way back in 1984. I can attest that every Mac OS version and every Apple application has had bugs and issues.
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