Working on issue with Friday server, will explain more after fix.
The root disk partition ran out of space. This is 2x 512G NVMe and shouldn't be storing emails. There is nearly 9TB available on the box where this data should have been stored.
This is clearing up. It seems that adjustments to the backup system may have caused this. More in postmortems channel in chat.mxroute.com.
Legal content allowed, don't be a dick.