Mail precedence header set to list

I am hand sending inbound emails from Gmail. But Discourse rejects with complaints that Precedence is set to list, and that the mail is auto-generated - but it is not: I typed it.

From the headers:

Auto-Submitted: auto-generated
Precedence: list
List-ID: The Jackrail | Test <test.jackrail.space>
List-Archive: https://jackrail.space/t/a-test-please-work/579
List-Post: mailto:replies+591d4708be667d02b1c5826953683fba@jackrail.space

What is going on?