procmail insists in writing to /var/mail in macOS Sierra

Solution 1:

The error message you see is basically harmless, but it's Procmail telling you that it cannot create the default inbox with your current permissions -- you need to be root to create an empty mailbox.

Creating it manually with the right permissions should resolve this issue.

sudo install -o $USER -g mail -m 0600 /dev/null /var/mail/$USER

procmail -v simply reports what the compiled-in defaults are; it does not examine your .procmailrc at all (and if it did, any nontrivial recipe file would contain a large number of mailboxes with different conditions for when to write to which).

"Fall off the end" means if you have a .procmailrc which does not tell Procmail to deliver to a specific mailbox and stop processing (such as yours), the final action will be similar to if the last lines in your .procmailrc were

:0:
$DEFAULT

which also illustrates what an unconditional Procmail delivering recipe looks like.

The log file snippet from Procmail you posted shows that Procmail is actually executing your .procmailrc just fine. The error message apparently happens while it's starting up, before it starts executing your .procmailrc (though I cannot repro this exactly, so this is mildly speculative).