Bounces from optonline

I'm getting bounces from one service provider and I'm not sure what the problem is or how to fix it. It seems like it's looking for some kind of reverse DNS (reverse DNS is set up for our domain) or it's objecting to the hostname?? I'm not a quite a noob but am a bit fuzzy on some of these concepts and I'm not sure what the right direction to take to eliminate these bounces is.

Here's the bounce message…

This is the mail system at host li52-167.localdomain.

I'm sorry to have to inform you that your message could not

be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can

delete your own text from the attached returned message.

The mail system

<xxxx@optonline.net>: host mx1.optonline.net[167.206.4.77] said: 550 5.7.1 We

do not accept mail from dynamic-IP hosts or generic/machine-generated

hostnames - you must get a custom reverse-DNS name from your ISP for your

host: li52-167.localdomain: xxxx@optonline.net (in reply to RCPT TO

command)

Reporting-MTA: dns; li52-167.localdomain

X-Postfix-Queue-ID: ACAF02120D

X-Postfix-Sender: rfc822; xxxx@mydomain.com

Arrival-Date: Fri, 1 May 2009 13:15:21 -0400 (EDT)

Final-Recipient: rfc822; xxxx@optonline.net

Original-Recipient: rfc822; xxxx@optonline.net

Action: failed

Status: 5.7.1

Remote-MTA: dns; mx1.optonline.net

Diagnostic-Code: smtp; 550 5.7.1 We do not accept mail from dynamic-IP hosts or

generic/machine-generated hostnames - you must get a custom reverse-DNS

name from your ISP for your host: li52-167.localdomain: xxxx@optonline.net