Sender blocked

Good day. Since last Friday, 17th of February, when trying to send answer I'm getting following error:

Error while processing the request! Unable to send message;Reason:554 5.1.8 Sender Address Blocked

I've already submitted several support requests, tried to contact you via chat in CRM several times, but to naught. What the hell your client should do to get unblocked? More so, I'd like to hear a reason for this as well.