/etc/mail.rc is in bsd-mailx 8.1.2-0.20160123cvs-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | set ask askcc append dot save crt
ignore Received Message-Id Resent-Message-Id Status Mail-From Return-Path Via Delivered-To
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