As far as I know that's the expected behaviour. I also saw it in practice, although I did not study the exact distribution in much detail. The reason for this

As far as I know that's the expected behaviour. I also saw it in practice, although I did not study the exact distribution in much detail. The reason for this behaviour would be that you would normally use this for load balancing. You can normally get the same behaviour by setting one MX record to a local A record (mail.example.com), and then serving several IPs for that A record in a round-robin/random fashion.