Why are incoming messages being automatically discarded?

From the post at https://mail.python.org/pipermail/mailman-users/2016-July/081004.html, the handlers that do the discarding and their reasons are:

MimeDel

• content filtering removes the entire message and filter_action is other than Reject

Moderate

• a post is from a moderated member with member_moderation_action Discard
• a post is from a non-member in discard_these_nonmembers or with generic_nonmember_action Discard

Scrubber

• if the top level MIME type is text/html and ARCHIVE_HTML_SANITIZER = 0 (not the default, and this only applies if scrub_nondigest is Yes)

SpamDetect

• when a message matches KNOWN_SPAMMERS
• A post is From: a domain with DMARC p=reject (or maybe quarantine) and dmarc_moderation_action is Discard
• a message matches header_filter_rules with Discard action or with Reject action and the message is to -owner

Note that Mailman >= 2.1.19 logs the name of the handler in the vette log discard entry.