CODE ERROR 554 5.1.8

CODE ERROR 554 5.1.8

We are a design agency and we need the email to prospect potential clients, that's why we send a lot of emails, we are not sending spam.
An error called ERROR CODE :554 - 5.1.8 Address Blocked address appears.
We cannot be without email, if the problem is not resolved, we will cancel the service.

Can anyone help?