Problem With Exchange Server – HELO and DNS MX Settings

Hi.
I administrate a server with windows sbs 2003 premium for 80 users. In this server is running a exchange server.
When some users send email wiich domain is in the same ISP that I, it comes back with the following message:
: Recipient address rejected: Mail appeared to be SPAM or forged. Ask your Mail/DNS-Administrator to correct HELO and DNS MX settings or to get removed from DNSBLs; MTA helo: my-company-domain.com, MTA hostname: ip.static.rev.my-isp.pt[My-IP] (helo/hostname mismatch).
I read some articles about the error 550 5.7.1 in MS but still having no clue what so ever how to resolve the problem.