This folder stores persistent application data like unprocessable emails, and must be shared in cluster setups to be available for all nodes.
Possible subfolders (depending on system configuration and usage):
var/spool/unprocessable_mail- stores emails that could not properly be imported
Just for reference, Zammad 6.1 and earlier did also store here:
var/spool/oversized_mail- emails that were rejected as too big