Unable to send message;Reason:550 5.4.6 Unusual sending activity detected. Please try after sometime

Unable to send message;Reason:550 5.4.6 Unusual sending activity detected. Please try after sometime

Hello

i'm unable to send any email because i keep getting this error

Unable to send message;Reason:550 5.4.6 Unusual sending activity detected. Please try after sometime

i have literally sent less than 10 emails today

i'm not sure why i'm getting this