This is clearing up. It seems that adjustments to the backup system may have caused this. More in postmortems channel in chat.mxroute.com.
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.
MXRBL has quickly become a powerhouse at blocking spam. The volume of spam we're blocking each day, without false positives, is incredible. Even if you don't use MXroute, if you host your own mail servers, you are free to use it: https://mxrbl.com
Legal content allowed, don't be a dick.