Does Hotaru use sendmail to send this email? I was poking around in my host's knowledgebase and came across this:
SMTP fails when sending through mailing list, but no error logs are generated.
Due to recent changes in our firewall, we do not support the origination of an MTA from our server. This change was brought about to eliminate the possibility of spam originating from our servers across port 25. Alternatively, you can use sendmail, located at /usr/sbin/sendmail, with the mailing list script.