You’re right: I meant the System Wide tracker.
Just to confirm for any others, though, my problem is the same as yours: my clients’ emails are bouncing with the same error.
Unfortunately, there wasn’t a better category through which to report this problem via the support page. I used the best fit, and when I finished, the system informed me it was already being tracked as an “outage”.
I’ve just sent a message via the Support Request as well.
I’m assuming (hoping?) that DH is aware of the issue and trying to address it…