Yes. Find the source of the problem and stop the spam. Once the spam is stopped and the problem that causes it has been corrected create an account at SORBS (Spam and Open-Relay Blocking System) . Open a support ticket and let them know both what the problem was, how it was corrected, why it won't happen again, and don't forget to include the IP address in the ticket. As long as you appear to have been a good sender with a one time problem SORBS should delist you.Anyway to fix this? Im trying to get this customer off the blacklist but have no luck.
I removed the IP address to protect your clients privacy (since it's me creating a thread from an email), but it's worth noting it's a good thing that their reverse DNS TTL is 1D (more than 14400 seconds). The SORBS minimum TTL for getting delisted is 14400 seconds.