out of place mx records.

[Mike Bernhardt]

I'd like to suggest an alternative reason for the presence of those records:
The Perl script H2N will install them by default for every single host in
the zone file, unless you use the -M option to suppress their creation.

Obviously this has nothing to do with the value, or lack thereof, of those
records. But in answer to your original question, it may be that the only
reason they exist in your environment is that your predecessor used H2N with
minimal customization to generate the zone file.