Relaying denied error

I was sending messages to my client just fine for the last few weeks. Then all of a sudden I send her an email and get this reply:

Recipient address: (left blank for confidentiality)
Reason: Remote SMTP server has rejected address
Diagnostic code: smtp;550 5.7.1 <(left blank for confidentiality)>... Relaying denied: You must check for new mail before sending mail.

What should I do about this?

Thanks!