DNS PTR record

[Abo Sous <abussous () gmail com>]

Hi List,

i am digging a mail server and a router on my domain; i get the
following results:

1- the dig domain.com MX gives me:
;; QUESTION SECTION:
;domain.com                                 IN            MX
 ;; ANSWER SECTION:
domain.com.                  20778      IN            MX          10
mail.domain.com.
 ;; ADDITIONAL SECTION:
mail.domain.com.                          20778      IN            A
          y.y.y.230

2- the dig -x y.y.y.231 (the router) gives me:
;; QUESTION SECTION:
;231.y.y.y.in-addr.arpa.        IN      PTR

;; ANSWER SECTION:
231.y.y.y.in-addr.arpa. 21495 IN        PTR     mail.domain.com.

obviously, the output from step number 2 is wrong

my question is: where could this error possibly lead to? what are the
vulnerabilities / security issues that might arise from this?

thanks in advance,

-A/S