I am having a problem with the email bounces to a subdomain only account hosted on the server with no local mail exchanger. I will appreciate if anyone can offer some insight.

First some info that may be required to understand the issue.

There are two IPs assigned to server both with rDNS setup:
venet0:0 a.b.c.56 x.com
venet0:1 a.b.c.57 vps.x.com (Main shared virtual host IP, main ethernet device)

DNS records vps.x.com has an A entry:
vps.x.com IN A a.b.c.57

There is also a corrosponding entry in x.com's DNS records:
x.com IN A a.b.c.56
vps IN A a.b.c.57

WHM >> Main >> Account Information >> List subdomains show:
Domain: x.com
Subdomains: vps.x.com

Problem:
There is an accout hosted that is a subdomain only, say, subdomain.y.com. The email for this account is handled by the remote exchanger where the y.com is hosted so is setup as such. In its DNS records there is only NS records, which point to remote DNS servers, and an A entry:
subdomain.y.com IN A a.b.c.57

For email routing of the domain "Remote Mail Exchanger" is selected.

This account has an application in which the visitors can register and a confirmation email is sent to the account holder and the registrant. This works fine. However if the registrant provides a non-existent email address then a bounce message is queued in the mail queue.

from [bounce]
received from mailnull by vps.x.com
for subdoma@vps.x.com

where the subdoma is the name of the account of subdomain.y.com

Any ideas how can I resolve this issue? Thanks.