550 5.4.6 Unusual sending activity detected. Please try after sometime

550 5.4.6 Unusual sending activity detected. Please try after sometime

Hi, I am receiving this error message when trying to send my emails. The only reason I can think why this is happening is my previous two emails were bounced back to me due to a non working mailbox error. I have followed the online links for unblocking but it says there are no blocks on my account. How and when can I get my email working again to send emails?

Thanks,