duh.

core/admin/mailu/configuration.py

@@ -145,7 +145,7 @@ class ConfigManager(dict):
         self.config['SESSION_COOKIE_SAMESITE'] = 'Strict'
         self.config['SESSION_COOKIE_HTTPONLY'] = True
         self.config['PERMANENT_SESSION_LIFETIME'] = timedelta(hours=int(self.config['SESSION_LIFETIME']))
-        hostnames = [host.strip() for host in self.config['HOSTNAMES'].split(',', 1)]
+        hostnames = [host.strip() for host in self.config['HOSTNAMES'].split(',')]
         self.config['HOSTNAMES'] = ','.join(hostnames)
         self.config['HOSTNAME'] = hostnames[0]
         # update the app config itself