Sometime this morning Igloo, the server that hosts the mail spool, just stopped serving files in /mail via NFS. It wasn’t a case of NFS dying altogether as other files systems still worked.
I wasn’t able to troubleshoot much further because when I got to the co-location facility, the console was dead and it would not allow remote logins. I was forced to power cycle to reboot after which everything appeared normal.
These machines were recently upgraded from Ubuntu 18.10 to 19.04 and that upgrade included the first 5.0.0 Linux kernel. I have had issues with that kernel on other machines and have resolved them by compiling 5.1 which appears to be more stable. I am doing that for this machine now which means there will be an additional interruption when I boot into the new kernel.