The mail server is fixed, though I still have to check NFS mount points of various servers, however, the CAUSE of the issue with mail was not, as I had suspected, the new kernel.
Something has changed the behavior of the system such that the order of data in nsswitch.conf is being ignored, and if there is a conflict in NIS UID verses local password file, the NIS overrules. As it happens, we had a postfix entry in NIS from a gazillion years ago when postfix was running on SPARC servers and that was conflicting with the local password file. The order of data in nsswitch.conf was files then NIS so it should not have, and further it did not show in ls, the output of ls was correct, but postfix interpreted the NIS version.
I booted off the old kernel and it still exhibited the same behavior so it was not caused by the new kernel as I had suspected. Removing the entry from the NIS database, which is no longer legitimately used anyway, resolved the issue so that the server works properly, still the issue remains that nsswitch.conf is being ignored in some contexts and this is not good.
If there is a plus side to any of this, I had to reboot the mail server numerous times during the trouble shooting process, and after doing this I checked all the servers for proper NFS mounting of the mail partitions and every single one of them mounted correctly. So it appears one thing that finally got resolved in 5.13.4 is NFS mount handling. This has been a bug that has plagued his since the beginnings of Linux and it appears finally put to rest. This will make kernel upgrades less problematic in the future.