MX record IP address instead of hostnames

[Lear, Karen (Evolver)]

Can anyone tell me why my MX record for the coop-uspto.gov domain are IP addresses instead of hostnames?

[klear at dns1 conf]$ nslookup
> set type=mx
> coop-uspto.gov
Server:         10.240.11.20
Address:        10.240.11.20#53

Non-authoritative answer:
coop-uspto.gov  mail exchanger = 5 151.207.128.23.coop-uspto.gov.
coop-uspto.gov  mail exchanger = 5 151.207.128.22.coop-uspto.gov.

Authoritative answers can be found from:
coop-uspto.gov  nameserver = dr-coopdns-1.coop-uspto.gov.
dr-coopdns-1.coop-uspto.gov     internet address = 151.207.240.200

Here is the zone file for coop-uspto.gov
[klear at dr-coopdns-1 named]$ more db.coop-uspto
$TTL 7200
@       IN      SOA     dr-coopdns-1.coop-uspto.gov. nmb.uspto.gov. (
                2011060708      ; serial number yyyy/mm/dd/## format
                10800           ; refresh after 3 hour
                3600            ; retry after 1 hour
                604800          ; expire after 1 week
                86400   )       ; minimum TTL 1 day

        IN      NS      dr-coopdns-1.coop-uspto.gov.

localhost       IN      A       127.0.0.1
dr-coopdns-1    IN      A       151.207.240.200
dr-idns-1       IN      A       151.207.128.33


;COOP-USPTO MX record
coop-uspto.gov. IN      MX      5     coop-mbxhc-0.coop-uspto.gov.
;coop-mbxhc-0    IN      MX     5     coop-mbxhc-0.coop-uspto.gov.
;coop-mbxhc-1    IN      MX     5    coop-mbxch-1.coop-uspto.gov.
coop-uspto.gov. IN      MX      5       coop-mbxhc-1.coop-uspto.gov.
mailer          IN      A       151.207.128.131

;www
www             IN      A       151.207.128.134
@             IN      A       151.207.128.134
coop-uspto-srv1 IN      A       151.207.128.134
coop-dc-0       IN      A       151.207.128.20
coop-dc-1       IN      A       151.207.128.21
coop-mbxhc-0    IN      A       151.207.128.22
webmail         IN      A       151.207.128.22
webmail         IN      A       151.207.128.23
autodiscover     IN     A       151.207.128.22
autodiscover     IN     A       151.207.128.23
coop-mbxhc-1    IN      A       151.207.128.23

Thanks,
k
>
Karen Lear
Evolver EUS - Network Operations
Phone:  571-272-5314
email:   karen.lear at uspto.gov

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