Mailman can create a non-ascii or improperly RFC 2047encoded From: header.

[Mark Sapiro]

When creating the new From: header for a message with from_is_list or dmarc_moderation_action set to Munge From or Wrap Message, if the incoming From: has no display name so Mailman gets the display name from the poster's list member data, and that name contains non-ascii, Mailman puts the non-ascii display name in the new From: header and subsequent processing can see the non-ascii and attempt to RFC 2047 encode it, but do that improperly.