ERROR_CODE :550, ERROR_CODE :Rejected sender

ERROR_CODE :550, ERROR_CODE :Rejected sender

Dear Zoho-Team,

I try to send an email to a person (an official one) with whom I already communicated before. Unfortunately I get now this error when I try to send a Mail. In the last days I got this error message several times, while sending emails to different people. I am not sure, what is exactly wrong. Can you help me please?

Best
Vika