I have a few domains on CloudFlare which are intended for websites only. Accordingly they have A records and nothing else. The A records point to my rented server at online.net, which runs Debian Buster. I don’t believe that server should be able to receive SMTP mail; certainly I have not intentionally set it up to do so, although it is set up to send mail using sendmail.
Anyway, I am interested to know what happens if someone attempts to send an email to such a domain. My understanding of DNS is that absent an MX records, delivery should be attempted to any A records. However, with CloudFlare in place (proxying the DNS) that presumably shouldn’t be possible as the A records are blocked to the world. I would have expected the email sender to receive a bounce message from his mail server? In my case, I tested by sending to an arbitrary user @<domain.with.no.MX.records.com> using a gmail account, but I have had no bounce message.