blacklisted_rcptto_emails: %w[c@c.com d@d.com] blacklisted RCPT TO emails not_registered_emails: %w[e@e.com f@f.com] Not registered (non-existent) RCPT TO emails response_delay_helo: 2 HELO response ...

For details of SMTP itself, see RFC2821. You must open a connection to an SMTP server before sending messages. The first argument is the address of your SMTP server, and the second argument is the ...