unattended-upgrades uses wrong hostname after switching from msmtp to Postfix

I recently switched from using msmtp to Postfix as MTA on one of my servers. unattended-upgrades used to send notifications with the content of /etc/hostname (webserver.example.org), but now it uses the base domain for which the machine serves a website (example.org). Postfix itself is configured to use the full hostname (myhostname = webserver.example.org), which is reflected in the headers, and /etc/mailname also contains the full hostname. Where does unattended-upgrades pick up the base domain, and why does it do so suddenly after changing to a different MTA?