It seems templates can be configured in the filesystem or via postorius. When I place a footer template in the filesystem it worked fine. Then without adjusting any configuration, without removing that, I tried to use the postorius control panel to add a footer template, which ends up in the database, both the core and web databases.
But emails stop getting sent out. Emails were posted into Hyperkitty, but not mailed out via postfix.
Whatever system logs I have access to, I have checked. There are no apparent errors anyplace. Perhaps I didn't look in the right places.
The problematic email arrives, is posted to Hyperkitty, but isn't emailed out to subscribers. As if the template broke something.
I remove the template from the postorius UI.
Those queued emails are instantly sent!
The template said "test content". No variables.
I checked on the server for "queued emails" and did not find those either, in postfix or mailman. Mysterious. The phenomenon is repeatable on this server. Add a footer template, emails stop.