/var/permission

from link, /var/mail should have correct permissions 2775 root:mail

The first digit "2" represents the setgid (Set Group ID) special permission:

2 = setgid bit enabled
When set on a directory, new files created within inherit the directory's group ownership
This ensures all mail files created in /var/mail automatically belong to the mail group

The s represents the setgid bit. It appears in the group execute position: