Now let's take a look at the ~40% (approximately 81 million) of domains _with_ MX records.
Most domains have between one and five mail exchange records, but of course there are outliers: a few hundred domains have >10 MX records, and some domains even have over 100!
The ever so aptly named everymailbox.com domain has 398 MX records, whiteinbox.net has 253, and rm02.net has 235.
All of these MX records have the same priority, suggesting they are trying to aim for some DNS round-robin load balancing here.