A long established contact in Germany is having their emails to me blocked.  No problem with their other emails.  The failure message they received says that their IP address has been found on industry IP blacklists (Spamhaus/Invaluement/ReturnPath)

I have been advised that my email provider (Windows Live Mail) is blocking their email provider and to advise our email providers that legitimate mail is being blocked.

Any assistance would be appreciated.  Thanks.