Guerrilla SMTPd is minimalist, event-driven I/O, non-blocking SMTP server. The design goal and purpose of Guerrilla SMTPd is to receive large volumes of email and nothing much else. It happily gobbles down millions of emails every week on a single processor system.
new_mail table was missing an ip_addr key; not having gm2_setting would make the example my_save_email() function fail writing to MySQL.