Connection to "process /usr/sbin/sendmail -bs" has been closed unexpectedly.

Found I needed to set the Default Transport to my SMTP sender. Embarrassing. 8 (

You've just changed it from sendmail to SMTP transport.

For anyone wondering how to get it working with sendmail, you need to customise the sendmail command. For example, this worked for me:

$settings['mailer_sendmail_commands'] = [
  '/usr/sbin/sendmail -t',
];

It seems that the default of /usr/sbin/sendmail -bs is not supported on many servers including Plesk and Platform.sh.