As noted on a number of complaints across Apple related sites, the newest update of macOS (10.14.4), which was released a few days ago, causes issues with authentication when attempting to sign into a user’s Gmail account. Users are reporting receiving an endless loop, bouncing between macOS and Google’s sign-in page in Safari.
Because of this, it is recommended not to upgrade to OS 10.14.4 if you use Gmail within your Mail application until Apple releases an update and it’s verified as a fix for the issue.
It appears this is a verified issue per discussions on the Apple Support site. Attempting to use a different browser such as Chrome does not succeed as a work-around, as the Mail application interacts directly with Safari in this process.
Apple Mail can also exhibit other intermittent issues for some users, such as not showing a message as replied-to where a user had just replied. If you are noticing any odd issues such as this, restart your Mail application.
You can read more about the Mail/Gmail 10.14.4 issue here. As always, if you need assistance, we’re here to help!