Dave,

 > Do I take it from your timings that there was a
 > second outage much later in the day?

It looks like that to me.

First thing in the morning Pacific (UTC-0800) I found that the copy of
Thunderbird I'd left running overnight was no longer authorized to
access my Gmail accounts. My receipt of messages from Groups.io via
those accounts was not interrupted at that time.

In the mid-afternoon my more active Gmail address went on Bouncing
status and I missed the receipt of several messages in both GMF and
Beta. I also missed the Bounce Probe (that I assume was attempted) and
only discovered my bouncing status because I visited the web site.

In GMF's Activity log the bouncing of Gmail accounts began around 2:45
pm Pacific and continued until about 3:41 pm.

The possible connection between the two events is that the bounce reason
was "The email account that you tried to reach does not exist". Not
exactly an authentication issue (inbound rather than outbound), but
still related to my Gmail account identity.

Shal