Application.ItemOperationException: Cannot send message (Email recipient not accepted by the server.)

Sending messages: Recipient address not accepted due to the following reason: 09:41:00 MailClient.Storage.Application.ItemOperationException: Cannot send message (Email recipient not accepted by the server.) The address is valid as it works on other systems, email isn’t full. I don’t know what to do, guess I will dump eM Client!