How is email able to reach this person despite no MX record?

Question

I have an email address for someone, where the hostname to the right of the @ sign is "mail.gongin.com.tw". When I try to do an MX record lookup on mxtoolbox.com, it reports "No Records Exist". Windows nslookup gives some information back but I don't see any "mail exchanger" data in the nslookup results. I've also tried an MX record lookup via this mail component and it too finds no MX records.

I sent two test emails to this person, each from a different email service. Both emails reached him and he replied to me.

How are these emails able to reach him? I imagine there must be some extra lookups that the sending mail servers are doing to find a mail server to deliver the email to, and if so, would like to know how that works.

Answer 1

If a domain has no MX records, delivery must be attempted directly to the host pointed to by the domain's A/AAAA records, as per RFC 5321 § 5.1:

If an empty list of MXs is returned, the address is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host.

mail.gongin.com.tw has one A record pointing to a server that accepts SMTP connections. (Which is expected, since the server is named "mail", after all.)