Google Mail Servers Refuses To Send To


(Apologies if this is not the right place... do not know where else to put it)
A few correspondents (coming from gmail) have alerted me in the past days that sending emails to my address bounce with:

Technical details of temporary failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain by [].

The error that the other server returned was:
554 5.7.1 Service unavailable; Client host [] blocked using; Currently Sending Spam See:

I tried going to and unblock the domain, but no luck.
Any clues on what to do? Who to call :) ?