A growing number of Mac users have noticed that after updating to macOS Mojave 10.14.4, the default mail client is unable to sign into their GSuite account.
After updating to macOS 10.14.4, most users received a notification from the Mail app that they need to reauthenticate their Gmail/GSuite account. This essentially requires them to sign into their Gmail/GSuite account again and be done with the issue. However, due to a bug, users end up in an endless sign-in loop where after a successful sign in, they are again asked to enter their GSuite account password.
Below is how the issue was explained by user Ken Graham over at Apple support forums:
This issue is UNIQUE to Mojave 10.14.4 as it did not exist with 10.14.3 and it does not exist on another computer running an earlier version of Mac OS X.
The issue puts me in an endless loop. I get a notification “Google requires completing authentication in Safari.” This takes me to a Sign in dialog where I enter email account and password. Password is correct and accepted, after which I’m back in the loop again asking to enter Google Password.
Gmail account holders are able to resolve the issue by simply signing into their account post the macOS Mojave 10.14.4 update. It is only GSuite account holders who are ending up in an endless sign-in loop.
The issue was seemingly also present in the final beta of macOS Mojave 10.14.4 and despite being reported, Apple did not fix it in the final release of the OS. If you are affected by this bug, you can use a third-party email client like Spark or Airmail until Apple and Google get around to fixing the bug.[Via Apple Support Forums]