First off, thank you for even viewing my problem. It's causing me serious anxiety and any help offered is so greatly appreciated.

The problem I'm having is with some local mailboxes not getting mail in the event that other local mailboxes were included.

For example, mail sent to:
ok_user@localhost.net = delivered just fine

afflicted_user@localhost.net = delivered just fine

ok_user@localhost.net AND afflicted_user@localhost.net = delivered only to ok_user@localhost.net regardless of order of email addresses, or method (cc, bcc, multiple to: fields)

The /var/log/mail/current shows the status as sent and queued, but it never makes it to the mailbox.

Again, I appreciate anyone actually reading this problem and if you could reply, I would be immensely grateful.
-Paul