Email "MTA poor reputation" error

Has anyone come across the following error message when using icloud mail?

"Your message cannot be delivered to the following recipients:

Recipient address: specificemail address quoted

Reason: Rejection greeting returned by server.

Diagnostic code: smtp;554-lonmx1.mail system quoted Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means.

Remote system: dns;lonmx1.mail system quoted

For security reasons I have removed the specific email address used, and also the server information but essentially where I show "mail system quoted" there is a reference in the error message to the server I was sending mail to. This problem has only started since 8 April 2013 and even then only with certain email contacts.

Any ideas why this has suddenly started happening?