I experienced that today - late at night, I was unable to control all lights (I have 116 devices), and my 2 Hue Bridges bridges (Inside Lights and Outside Lights) all showed this:

The following message to <support.philipslightingnordics@signify.com> was undeliverable.
The reason for the problem:
5.x.1 - Maximum number of delivery attempts exceeded. [Default] 450-'4.7.26 Service does not accept messages sent over IPv6 [2a03:dc80:0:f136::208] unless they pass either SPF or DKIM validation (message not signed) (S825). [DB5PEPF00014B8E.eurprd02.prod.outlook.com 2023-12-08T11:24:52.913Z 08DBF6F3BDF88AA9]'

The following message to <support.philipslightingnordics@signify.com> was undeliverable.
The reason for the problem:
5.x.1 - Maximum number of delivery attempts exceeded. [Default] 450-'4.7.26 Service does not accept messages sent over IPv6 [2a03:dc80:0:f136::208] unless they pass either SPF or DKIM validation (message not signed) (S825). [DB5PEPF00014B8E.eurprd02.prod.outlook.com 2023-12-08T11:24:52.913Z 08DBF6F3BDF88AA9]'

The Bridges bridges seemed fine, and I (beside panic) started to investigate:

...

https://www.online-tech-tips.com/smart-home/philips-hue-lights-unreachable-7-things-to-try/

Power off Bridgescycle bridges

Did nothing at all

Rebooting network Router

...

I just got some double entries, and the restore seems to be on ment for a factory resat bridge.

I tried the above a few times, and nothing changed.

After aprox. 1,5 hours I took the power to the entire house, and after the bridges came onup, the Outside Lights have bridge had most bulbs available again, but many sensors were not reachable.

So I went to work for 8 hours..

...

When I came home, I took the power to the entire house again, and after the bridges came onup, aprox. 80% of the bulbs on the Inside Lights bridge were reachable - and all sensors seemed ok again.

In the evening I started to power off cycle the power sources to for the bulbs that was still unreachable, and slowly the they came back in the app as reachable.

The next day, I took the power to the entire house again, and 3 bulbs were still not reachable. I took them out of the ceiling sockets and put them into a lamp, one by one - 2 on them actually came to life again and the last just had to be reset (power off 5 seconds, power on for 5 seconds, do that 5 times - when the bulb flashes, its resat).

So all is good now :-)

5 Days later

From the original issue occurred - One Hue brigde (Outside Lights) has all bulbs and sensors as unreachable again, the other (Inside Lights) is fine.

Tried to power of the bridge and the power to some bulbs, with absolutely no effect. Even almost stranger, I can see that the sensors on the brige became unavailable in Home Assistant over a period of time, its not like just - all unavailable suddenly at the same time ...

I have disabled Software updates for both bridges.

So, this afternoon I tried to change ZigBee channel - no improvements. But it was a mistake to do it.....

Then I power cycled the entire house, and now 4 sensors is back (2 motion, 2 window/door), the rest is still unreacable.

So I took down the 3 "Terasse" bulbs

I could not reset them in their outdoor socket (10 sec with the dimmer) - went fine inside

All 3 were reset, deleted in the app and rediscovered

So I took down the 3 "Front Door" bulbs

2 of them just worked inside instantly (really weird as I had changed ZigBee channel)

The 3rd needed a reset, dould only be rediscovered via serial number...

So, now I am even more confused.....

Later that evening...

I changed Zigbee channel on the Outside Lights back to 25, and bulbs and sensor just came online within seconds...