Mac Mail App Broken for Some Gmail Users in macOS Mojave 10.14.4

[edit] Google has now fixed the issue so the following workaround is no longer necessary.

i'd definitely be interested, i sometimes use charles.

Okay, so I've tried to make this as easy as possible. We're going to use Charles to rewrite a response from google to include the email address which is for some reason missing in google's own response. This is a one time setup only needed for verification.
[LIST=1]
* Download, install and run Charles ('https://www.charlesproxy.com/download/latest-release/').
* If this is your first time using Charles it should prompt you to authorize automatic proxy configuration. Do this. (Otherwise just make sure the macOS proxy is enabled and working)
* From the Help menu choose SSL Proxying > Install Charles Root Certificate.
* Once the certificate is installed, find it in your keychain (type Charles into the search), open it, expand the Trust section and set to Always Trust. You will be prompted for your password when closing it.
* Back in Charles, from the Proxy menu choose SSL Proxying Settings.
* Add a new location with Host: people.googleapis.com.
* From the Tools menu choose Rewrite and click the Add button.
* Add a new location with Host: people.googleapis.com.
* Add a new action with the following details...

* Type: Body
* Where: Response
* Replace Value: {"names":[{"metadata":{"primary":true},"displayName":"Your Name"}],"emailAddresses":[{"value":"your@email"}]}
* (Make sure you put in your name and email address)

* Once you save this, we're all set. Go to your system Internet Accounts or (Mail) and authorize your account.
* If everything thing is working, you can quit Charles and feel free to delete the certificate from Keychain.

That’s not a bug. That’s a feature.

I have a fix! Use iCloud mail :)

Eh, it’s whatever. I’m not worried. Haven’t kept anything of importance in my gmail account in a while, just annoying subscriptions.

Having been in technology all my life - bugs happen as much as we'd like them not to - all things are changing all the time. My issue is if this was discovered during Beta why wasn't it fixed before production. That is what Beta testing is about - having a large population testing a large variety of combinations, etc.
I have a few other issues like a game I play not working but it's last update was in 2012 so I probably should be surprised it was working this long. Apple should require that if something is still on the App store that the developers test during Beta and certify works or it gets pulled from store.
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Never thought I'd say this, but Microsoft is looking better and better these days'. They now seem to be the more stable of the bunch.

Run windows 10 machines also and I'd never make that claim. It is no better than it ever has been

I first noticed the issue on February 4th. It got slightly better with later betas.

https://forums.macrumors.com/threads/apple-seeds-second-beta-of-macos-mojave-10-14-4-to-developers.2168389/#post-27064233

I can verify the same. During beta testing of 10.14.4 I ran into the inability to log into Gmail. To solve it I first Quit Mail. I went to System Preferences/Internet Accounts, highlighted my Gmail account and deleted it with the minus (-) sign. I then Quit System Preferences. I then went back to the same setting again and added my Gmail account with address and password. I then rebooted for good measure. The problem didn't return. I recall that fiddling around in the Mail app itself solved nothing. Internet Accounts is what got messed up.

There have been some quirky little behind-the-scenes changes to macOS in 10.14.4 that have broken various things. I'm currently trying to sort out with a developer what Apple did to break the Command-C command when using ANY version of CopyPaste Pro. There are several little quirks in the latest version of Safari, evident if you visit YouTube and attempt to use the video playing interface. My experience with Apple is that this small stuff is of no interest to them, entirely fitting with my theory that they've failed to scale the Mac computer side of the company as the company has grown. I can toss out a dozen further examples. I find it all very annoying and wish they'd pay close attention to Mac again and return to focusing on the customer rather than whatever their distraction-of-the-day may be. :-(

We use GSuite on the company and a this sucks

This has happened before, back in October and the problem was on Google's end.

It may be a coincidence that this happened right after the update, but it's definitely a case of deja vu.

https://forums.macrumors.com/threads/internet-account-google-failed-to-authenticate.2146470/

I have a Fix. Use ProtonMail.

I have an idea. Why don’t people stop telling others to use mail service ‘x’ in defense of Apple’s blooper. It’s getting really pathetic always seeing people defend Apple when they screw up. How does this help make them a better company if they aren’t being held accountable? Workarounds do nothing but weaken an already splintered company.

Glad I haven't updated my personal MacBook Pro, in case this happens to me. Nice call on this one MacRumors. Now I'll just wit for an OS update before installing.