Without an MX record it falls back to...
You can find this in Wikipedia:
RFC 5321 sec. 5 states:
SMTP clients must look up an MX record;
if (and only if) no MX record for the domain is present, treat the domain as if it had an MX record with the given domain as the target hostname and a preference value of 0
perform A or AAAA lookups as required to determine the IP address of the target hostname
Biting the hand that feeds IT
