this usually means that your isp doesnt like the address that mail is coming from (i.e. your IP) you may need to check that you have the correct pop and smtp settings.
Are you using ACT internet mail or Outlook? does it work via outlook?
If you are using ACT internet mail and your ISP requires a port other that port 25, this could be the problem and you would need to use OUTLOOK or use the procedure documented in this previous post
