[MM3-users] Postorius confirmation mails shown in uwsgi-error.log but not sent

I have struggled for hours but cannot seem to figure it out. After registering new user accounts in Postorius, the "confirm your address" emails don't get sent via SMTP (my MTA does not even log connection attempts), but their content is displayed in uwsgi-error.log.

I think I disabled developer mode in Core and Postorius:

$ mailman conf | grep devmode [devmode] enabled: no [devmode] recipient: [devmode] testing: no [devmode] wait: 60s

$ grep ^DEBUG settings.py DEBUG = False

Users can subscribe via list-join@domain.tld, including confirmations. Account registration using the WebUI works as well, it's just the emails that don't get sent. What am I missing, I wonder?

-Ralph