GMail on one Mac (only!) stopped working this week I have a GMail account, and I access it from two Macs, and an iPhone.  The iPhone and one of the Macs (10.14.x) still works.

On the other Mac (10.13.6), it stopped being able to receive mail a couple days ago.  The "Mail Connection Doctor" says "Trying to log in to this IMAP account failed.  Verify that the username and password are correct", and likewise for SMTP.

I found some ideas online and tried them, but nothing so far has worked:


Go to https://myaccount.google.com and turn on "less secure account access" (it was already on)
Go to https://accounts.google.com/b/0/DisplayUnlockCaptcha


I haven't changed my password in the past month, and I can still log in to GMail with a web browser.

When I go to "Accounts" in Mail.app and click my GMail account, it presents an OAuth-like sheet asking for my Google email/password.  When I complete the form, the sheet turns gray (empty), and only the "Cancel" button remains.

I found reports that GMail broke on Mojave recently, but my pre-Mojave system is the one that broke.

Any ideas for how to make this work again?

UPDATE: I've been clicking "Check Again" several times, and one time I got IMAP (only) to work, and it downloaded this week's email, but then it stopped working immediately.  I don't think I was doing anything special that time.

In the "Detail", I see:

250-smtp.gmail.com at your service, [my.ip.address]
{many more 250-prefixed lines}
AUTH LOGIN
334 VXNlcm5hbWU6
{my b64-encoded email}
334 UGFzc3dvcmQ6
*
501 5.5.2 Cannot Decode response {big_ugly_code} - gsmtp
QUIT


That's not my password -- it really just says * there (though I don't know if Mail.app substitutes that in the window for safety).

UPDATE 2: Calendar.app has all my latest calendars, for this same Google account, and syncs fine on this computer (though obviously not with SMTP or IMAP).
Do you have your account configured with your regular GMail account password or an "App Password"?  Do you have two-factor authentication turned on? 

I've had my home cameras configured for several years to send me emails via SMTP using my GMail with an App Password to email me if any motion is detected. I usually get several emails a day but this week suddenly it stopped working.  I generated a new Google App Password and even disabled 2-factor authentication but I still can't send any emails from my cameras.  Again... this has worked flawlessly for 2-3 years with no recent changes to my camera.

I encountered someone else having similar issues here: https://www.cnet.com/forums/discussions/outlook-365-no-longer-connects-to-gmail-smtp/

I'm thinking google must have changed something.