[exim] End of the DNS MX record ?

Today, I noticed 3 different and unrelated email addresses whose domain
name does not have a MX entry; 2 in one country and 1 in another
country.

A Telnet to each showed a functioning mail server (MTX) running Exim
4.80.

Is the MX record officially redundant if one has 1 MTA ?


--
Regards,

Paul.
England, EU.

Centos, Exim, Apache, Libre Office.
Linux is the future. Micro$oft is the past.